Okay, I'm hesitating to post this because it makes me seem way more curmudgeonly than I actually (think I) am. Oh well.
I should also say that at least for me, and maybe for other people, none of this applies if you're a close friend or close family member. It's more when acquaintances or (godfuhbid) strangers offer their advice that I blanch.
What you say: There are soooo many options for people who want kids!
What I hear: You're probably too stupid to figure this out, but you can procreate without having sex with a man!
What you say: But you'd be such a good parent!
What I think: I'd also be a good race car driver, occupational therapist, or professional shoeshiner. Natural predilection does not a destiny make.
What you say: Some people are too selfish to have kids.
What I hear: You are selfish and shallow. Unless you have kids. In which case all is forgiven. But I thought better of you. Now you just make me sad.
What you say: You could always adopt!
What I think: No sh*t.
What you say: Lots of lesbians are having kids these days!
What I think: Lots of lesbians are also chain-smokers, alcoholics, drug users, glue-sniffers, head cases, doctors, truckers, and couch potatoes. So?
What you (usually another lesbian) say: My mom didn't fully accept my partner and me until we had kids. But now that she has grandkids, we're closer than ever.
What I hear: Your mother will never fully love you until you procreate.
What you say: There are SO many children out there who need good homes.
What I think: So why didn't you adopt instead of having biological kids? Oh--you're scared you'll end up with a crack baby or a psychopath from a Russian orphanage who's never been held? But I should go for it? Thaaanks.
What you say: NO one thinks they want kids. Then they have them and they're glad they did.
What I think: Am I the only person in the world who's ever heard of cognitive dissonance?
What you say: Are you thinking of having a family?
What I think: So, me + DGF + slightly swollen canine ≠ "family?" Screw you.
What you say: You haven't lived a full life unless you have kids.
What I hear: Your life is invalid. There's only one way to redeem yourself, and it smells like diapers.
What you say: You may think you know what love is, but you don't really know what love is until you have kids.
What I hear: All your feelings are pathetic, shallow, and invalid--mere shadows of what they could have been. Alas!
Okay, so I'm being melodramatic, but you get the idea.
I actually don't think the pressure is nearly as bad for lesbian and gay couples who don't want kids, as it is for straight couples who don't want kids. People basically assume that opposite-sex couples are going to have kids, and that if they don't, it's because there's something biologically "wrong" with them. Instead of just getting asked, "Do you think you'll have kids someday?", people will ask questions like, "Do you think you're going to... start trying?"