What should be included in a disclaimer about the omission of gender-neutral language in a bachelor's thesis?

I am going to respond to various parts of your question step by step.

For those unfamiliar with German, I am going to add a brief description of the gender-neutrality issue at the end of this answer, because things are quite a bit more complex in German than in English.

German universities are very sensitive when it comes to gender-neutral language. Some professors mark it as a mistake if you use gender-specific terms instead of gender-neutral ones.

I am not convinced that statement is true for German universities in general, especially as there still is no single true agreed-upon consensus on what is a good way to navigate around the issues. Much rather than that, I can imagine that certain professors make it a point to enforce gender-neutral language (just like I could imagine some other professors to make it a point to enforce what they perceive as stylistically and grammatically correct language, even though it is in turn not considered gender-neutral by all).

While I don't want to be sexist, most variants of gender-neutral language in German are (in my opinion) verbose, disturb the text flow and make the text harder to read.

While in your Bachelor thesis, your stance on this issue carries little weight, this is by far not a rare opinion, and exactly one of the reasons why some people (including professors, see my paragraph above) vehemently oppose some variants of gender-neutral language.

This is why I want to put a disclaimer in the introduction of my thesis stating that I will not use gender-neutral language for the reasons mentioned above.

In my opinion, a disclaimer is a good idea, as it kind-of puts you on the safe side by making your intentions explicit. In fact, I would see the following benefits:

• It gives you some freedom to pick the style you'd like to write in.
• It might satisfy even some of the readers who would personally have chosen a different writing style.
• The mere existence of the disclaimer shows you care about the document as a whole, not only the specific technical aspects of your Bachelor project.
• It defines something that will be used later. In technical subjects, or at least subjects that are somewhat related to maths, choosing definitions in a convenient way, explaining them first, and then using them accordingly throughout the document, is a good practice.

Now, there is no guarantee these points will be picked up positively, but at least, there is some chances for it.

This notwithstanding, you should ask your advisor for their preferred style, if any, or check past theses (successfully) written for the same advisor.

Is such a disclaimer generally acceptable in a bachelor's thesis (or any scientific text)?

As meanwhile shown by the other answer, yes. In fact, most things that clarify the contents are acceptable in Bachelor, Master, and Doctoral theses.

If so, are the three points stated above both adequate and thorough to state my intention, or what should I add/remove?

The three points sound fine. I would indeed advise against expressing you are "not using gender-neutral language". You could try and focus on stating that some terms "can be understood in a gender-specific way".

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Remarks on issues with gender-neutral German

At least according to my impression, gender-neutral language is a much more controversial topic in German than in English (for some further illustration of this, also have a look at a related question on German SE):

In English, a large part of gender-neutral language is replacing exclusive uses of he or his. In German, in contrast, gender-neutral language primarily tackles any noun describing a person, because these usually come in a male and a female form, the latter of which is usually formed by adding the suffix -in to the male form.

This opens up two issues that make the topic so controversial in German:

• On the one hand, there is no general agreement that the male form is, in fact, not gender-neutral. It has commonly been (and still is) used when both genders were meant, and (as noted below) there often is no synonymous noun that is gender-neutral.
• On the other hand, there is no commonly agreed-upon solution for how to express things in an explicitly gender-neutral way:
  • For a start, truly gender-neutral nouns for persons are really rare in German. Replacements such as the English steward/stewardess -> flight attendant do not produce gender-neutral terms in German. (Curiously, the terms are sometimes copied from English nonetheless, so Germans have replaced Stewards und Stewardessen with Flugbegleiter und Flugbegleiterinnen.)
  • Offering both alternatives every time a person is mentioned can make a text hard to read (imagine a text about actors and actresses, or actors/-resses, or actorsResses, and imagine that split happening for every single noun referring to a person). Hence, this variant is opposed by some native speakers.
  • There is also the suggestion of using the neutral gender for all gender-neutral nouns. (In English, this would be akin to generally referring to persons whose gender is not specified as it.) While this sounds logical, it can also be perceived as disrespectful, as (unless the person noun specifically calls for neutral gender) using neutral gender on a person is like treating the person like a "thing".
  • Another approach is, as mentioned in the question, using a grammatical form that is (for many words) not normally used as a noun, but that does not change by gender, and that arguably has the same meaning as the role name - the present participle. If we transfer this back to English, a sentence such as "Participants pay attention to the instructor before their conversation with the student." becomes "Participatings pay attention to the instructing before their conversation with the studying." This is opposed by some native speakers as featuring abysmal style and bordering on being ungrammatical.

Therefore, a disclaimer expressing the author's intentions can be seen as preferable compared to any of the "gender-neutral" variants, all of which are controversial and cumbersome in some way.

Reasons against a disclaimer

If not, how can I go about not using gender-neutral language without having it possibly be regarded as sexism or influence my grade?

In my opinion, a disclaimer is not going to help you here.

Either the people reading your thesis are already fine with the generic masculinum - in which case you don't need a disclaimer - or they are not.

If they are not fine with it, your disclaimer is not going to convince them that it is fine to use the generic masculinum. People who are against it already know your arguments for it, and have dismissed them. Without a disclaimer, they will not assume that when you for example speak of doctors, that you actually mean that only men can be doctors. But with a disclaimer, they may think that you thought about the problem, but came to the conclusion that you will not make an effort to not exclude women in your writing.

Either way, a disclaimer just seems like rubbing it in: "I know you would prefer it differently, but I'm not going to do that".

Improvements of your disclaimer

Your first and third point are essentially the same. The third only adds that no offense is intended, which isn't really helpful, so you might as well combine your points:

Throughout the thesis, gender-specific terms may be used in order to ease the text flow. Whenever a gender-specific term is used, it should be understood as referring to both genders, unless explicitly stated.

I'd also be interested in how the explicit stating would work in practice ("männliche Ärzte"?), but that would be a different question.

Examples of disclaimers

If you do plan on using a disclaimer, I would just put it in a footnote, no need to draw too much attention to it. An example:

Es sind stets Personen männlichen und weiblichen Geschlechts gleichermaßen gemeint; aus Gründen der einfacheren Lesbarkeit wird im Folgenden nur die männliche Form verwendet _source

Here is another example, it's placed after the content overview:

Aus Gründen der besseren Lesbarkeit wird auf die gleichzeitige Verwendung männlicher und weiblicher Sprachformen verzichtet. Sämtliche Personenbezeichnungen gelten gleichwohl für beiderlei Geschlecht. _source

Your arguments for the generic masculinum

In case you are not aware of this, there have been studies regarding the readability of text without the generic masculinum, and it doesn't seem to have a negative effect. There have also been studies showing that readers to not process generic masculinum as referring to both genders.

Other solutions

Looking at your disclaimer, the generic femininum seems like an equally good solution. It doesn't disturb text flow, and it can be understood as referring to both genders. And if your professor actually does have a problem with the generic masculinum, it might be a good solution that makes you both happy.

And as you didn't mention it, there is always the gendergap (Student_innen), gender star (Student*innen) or binne-i (StudentInnen). They may be less disturbing to your reading flow than naming both forms separately (Studenten und Studentinnen).

A common approach is to alternate genders each chapter. Chapter 1 would default to female-specific terms. Chapter 2 would default to male-specific terms. Etc.

You could explain why you choose to not include both genders at every mention, and that you've chosen this alternative.