shit! how old is this chick if she has a 13 year old son? Shit, that would turn me off from the start. Didn't she consider that? That women that include their sons at an intimate moment is going to turn the guy off?
I find girls pitiful that think that any guy that doesnt jump at the opportunity to fuck is gay. Especially if the chick questioning your sexuality isn't all that attractive and has an annoying personality.
if she tries to insult you like that again, just say "why am I gay, because I can't get it up with you?" that will shut her the fuck up.