In the pilot episode we find out that Carol and Ross have split up because she is in fact a lesbian and has been carrying out a lesbian affair with Susan. Ross is truly heartbroken over this. Even more troubling, he finds out after the break up that Carol is pregnant with his child and she plans to raise the baby with Susan. Carol later gives birth to Ross's son, Ben.
Ross: Every day is Lesbian Lover Day.
Ross: (talking about the baby's surname) Wait a minute, why is she in the title?
Susan: Because it's my baby, too.
Ross: That's funny, I don't remember you making any sperm!
Ross: (inspecting the books at Susan and Carol's place) Wow, you guys sure have a lot of books about being a lesbian.
Susan Bunch: Well, you know, you have to take a course... Otherwise they don't let you do it.