OP I think you handled it well. And it's good she was comfortable confiding in you
And, rightly or wrongly, life is generally easier for people who conform to social norms
And it's easier for a woman to get pregnant if she's having sex with a man
And we all want our children to be safe, happy and have an easy life
So, i understand why you were upset
My advice =
(1) I wouldn't make a big issue of it, same way you wouldn't make a big deal if she told you she was straight
(2) not popular on here, but i do suspect some teens think they're gay because they feel more comfortable around people of their own sex. As they grow older, their views may change. I know several people who thought they were gay as teens but ended up in relationships with people of opposite sex
(3) don't make an issue of it. It's not positive or negative
(4) be open minded and supportive
Good luck