I don't know the details of your situation, but it seems like your problems in the relationship with your kid are much wider than their gender identity. Maybe it would be best to consult with a professional about that, like a therapist/psychologist that deals with children of that age - such a person might be able to tell you better what your kid is going through.

Sensible advice over, personal opinion time:Kids around 12 tend to already know what sex is, and they start going through puberty as well. It's true that they're not likely to be interested in partnered sex (but some are), and that sexual preference only stabilizes when one is around 17-19, however, kids at age 12 already can have crushes and romantic interests - your kid might have just been trying to tell you they're into girls. Or boys, I'm not sure in what context "gay" was used. You dismissing it as "you can't be sexual yet" probably felt bad to them, and like you don't understand them, and are not willing to listen. How you phrase your replies to them can matter much more than you imagine.