But it really should be a simple matter of saying "no". You don't owe anybody any explanation for why you're not attracted to them, and you can make anything up if you want. I do understand that for lesbians avoiding MtFs it's a little trickier because of the potential threat of violence.

It should, but it isn't. The threat of violence is still there. Even with normie straight girls in gay bars, they get aggressive when in groups and told to fuck off and keep their hands to themselves and start decrying you as sexist for having boundaries.

You can excuse yourself, but it won't prevent stalkers and their enablers.