Uh, does this help? https://itch.io/games/tag-femboy
I don't know if every developer who makes femboy games would remember to tag is as such since the tag doesn't automatically pop up when you're typing it. A developer might probably think folk won't look under the tags and opt to pick the ones that do pop up instead.
An alternative is looking using the Search option (https://itch.io/search?q=femboy) but the limitation here is that not every title appears when you're using this option. Some games that aren't already tagged do pop up though.
Anyway, good luck! Femboys seem quite popular, from what I've seen. I'm surprised they don't already have a Femboy Game Jam hosted. Maybe you could host one and get more femboy content put out!