Is it ethical to apply for a short-term grant with a partner/spouse/girlfriend?

Provided you are qualified and intend to do a good job under the grant, I see no ethical considerations at all. Many people (most?) applying for grants in other countries have a variety of goals, not all of which are relevant to the position itself. Wanting to live in Paris is pretty common, I'd guess.

There would be an issue if she is the decision maker on who gets the grant. In such a case she should recuse from the decision and pass it to others, such as a superior. But otherwise, there should be no problem.

There is the issue of propriety, and there is the somewhat separate issue of the appearance of propriety. Although I agree with @Buffy that from an ethical point of view all that really matters is that you are applying for the grant in good faith with the intention of actually carrying out the research you are proposing, the fact that the intended research is with your significant other certainly has the potential to create the appearance that, at the very least, you are mixing your personal interests with your stated professional goals in a somewhat unhealthy and perhaps ethically compromised way (and no, it’s not really comparable to “wanting to live in Paris” as in Buffy’s example - as evidenced for example by the fact that you were concerned enough about doing this to ask this question). And at worst, some observers who learn of this may be inclined to assume an unethical motivation and regard this as a (perhaps mild) form of abuse of the grant funding system. Whether they are correct or not is besides the point; people sometimes jump to incorrect conclusions, and you should protect your reputation.

My advice is to disclose the relationship and let the people in charge of the grant decide if this is a problem or not. If they are okay with it, no one can fault you for accepting the funding — you have a watertight defense in case someone down the road criticizes your actions. And if they aren’t, well, there’s a pretty strong case to be made that hiding that information with the intent of increasing your chances of receiving the funding would have been somewhat unethical. It’s maybe in a gray area of things that people do once in a while and usually get away with, but I wouldn’t recommend it, both because it’s ethically problematic and so you can sleep better at night.

Edit: your edit to the question, and some of your comments, give me the impression that you read my answer as questioning your motivation or criticizing you in some way. Let me be clear, I see no reason at all to question that your motivation is pure.

With that being said, you asked “are there any ethical issues”, and frankly there are some “ethical issues” with doing something that, while you privately know it is ethical, can lead other people to think that you are doing something that may not be ethical. As I said in a comment, we live in a world in which grant funding, and other taxpayer-funded resources, do get abused sometimes. By behaving in ways that other people may perceive as unethical, you risk hurting your own reputation and career, and also push the delicate equilibrium many parts of our societies are built on in a direction that (ever so slightly) encourages genuinely unethical behavior by other people in the future. Conversely, by disclosing the relationship you will be enforcing norms of good governance. From a social point of view, you will be helping academia function a little bit better.

It’s fine if you disagree with any of this, but talking about “tuning out” because of “judgmental glares” misses the point of what I’m saying. I welcome any sort of critique of my logic and reasoning as long as it’s itself based on logic and not on emotional arguments like you being annoyed about people being judgmental. For example, Buffy’s argument that society is better off if grant applicants are not turned down because of wanting to work with their significant others is a valid one (not completely compelling in my opinion, but at least valid, and based on logic).

Based on personal experience, I don't feel that there are any ethical violations in simply applying for a grant with a significant other, and I certainly don't feel that you are required to disclose this information in your application. I am aware of many academic "power-couples" in my field that co-PI large projects, including projects on federally-funded grants. I have read the entirety of a few of these proposals, and I have never seen indication that the collaborators are married/partners, including those who have different last names. In fact, as a reviewer, I would find this information extraneous, distracting, and even unprofessional, especially if you have a strong track record of collaborative work together already.

Where I DO feel that there are ethical issues is if a collaborator includes their significant other on grant proposals, student committees, or papers when they did not/will not contribute substantially. For example, if a graduate advisor forces their student to put their spouse on their graduate committee to help the spouse achieve tenure requirements. These are examples of nepotism that definitely raise ethical issues.