I think all gay men have male figures in their lives that they aren't attracted to. I'm sure that choosing a female for metta cultivation wouldn't be a bad idea, but they could also choose a male relative or close friend. I've used a close female friend of mine in metta bhavana often and it's never been a problem, as there is no attraction between us.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti Sutta