Is there certain things a gay/lesbian friend and a straight friend just can't talk...

depends entirely on the people involved. for example according to a gay friend of mine it's ok to talk about his sex life with me because i don't get grossed out or uncomfortable, but it's not ok with another friend of his because she's a prude.

Depends how open the people involved are and how well they know each other.

I don't think there is anything as a general rule, but some people may get uncomfortable discussing each others' sex lives.