If you have friends or family members who are prone to blurting out horribly awkward and insensitive things, then you might want to gently forward them this Salon post, which handily rounds up all the worst possible things you could say to a woman who likes women, including:
4. How do you know you’re a lesbian if you’ve never had sex with a man?
How do you know you’re straight if you’ve never had a dick in your mouth? Or vice versa.