1) Don't ask them how they have sex. (rude and no ones business anyways)
2) Don't ask them to have a threesome with you. (Rude. Never assume someone you barely know wants to sleep with you, much less sleep with you and someone else they barely know).
3) Don't ask them "what made you gay" or "since when were you gay?" (It just shows your ignorance)
4) Don't tell them "You're too pretty to be gay" (you're implying that lesbians are all ugly and that lesbians don't deserve attractive women)
And then there's the obvious things like, don't tell them they're going to burn in hell, that they're disgusting, that they're unnatural, that they are sinning ect ect
Those (the top 4) are commonly asked questions (or statements) where I live. Then again, my state is apparently like 2nd worst for LGBT people, so it's no wonder people where I live are so offensively ignorant on the subject.
When I told my best friend I was gay, she asked "since when" and it was an honest question. I didn't get mad, but I just can't believe people where I live are so ignorant on all this, like, many people still think you wake up one day and decide to be attracted to the same sex. It's okay though, my friend was cool with it and she has learned a lot since then lol.
When I told one of my guy friend I was gay he said "So does this mean you'll send me nude pics?" And I was just like WTF?? I didn't even understand his logic there TBH. I guess he thinks gay people all send nude picks willy nilly to everyone.
Anyways, lesbians are just normal people. Talk about whatever you want to, but try to avoid topics that divide people and start wars (abortion, religion, gay marriage, feminists ect ect). So avoid topics that you would avoid with anyone else.
Basically just treat them how you would any straight person. You wouldn't ask a random straight girl to have a threesome with you, or tell her she's too pretty to be straight. You wouldn't ask her how she has sex, or ask her "since when were you straight" lol.
Just be relaxed and treat them how you treat everyone else, if you do that you'll be fine.