How can I protect my credit card on sites where 2FA is not an option?

There's nothing you can do. Because if it's actually you using the web site, then fraud is obviously not an issue. The 2FA only helps if someone else is using a web site pretending to be you, and so nothing that you do on web sites will make any difference. You can't stop a fraudster who has got hold of your card details from using them on a web site that doesn't use 2FA.

Instead of seeing this from the point of view of the card, look at this from the point of view of your account's money (that after all it's what matters, right?).

So, how can you protect your money in such sites? Simple solution: using a virtual credit card. Most banks offer these kind of cards that you should always have with no funds. Then when you are about to buy something, just load the exact amount so after the purchase the card is again with no funds.