So, if we follow your reasoning, wearing a wig makes the MC gay?
Ladies and gentlemen who wear wigs, I'm sorry to inform you that you are gay men.
- What's more, the MC begins to develop female breasts (thanks to the hormones ingests). It's true that boobs it’s very gay.

- And finally, so far the MC has had sex with women only. There's nothing gayer in the world, the gay tag is must.

Sarcasm aside, I don't see what a "gay" tag would do here. There is no justification for such a request. And by the time the MC is fucking dudes, wig or not, she'll have already become a full-fledged trans woman (
which is practically the case at present) so still no need "gay" tag.
P.s.: However, since there are actions between women, you can ask for a "lesbian" tag.