In some cases, having a gay parent has made it easier for them. Now some of those children are teen-agers and young adults who are realizing that they, too, are gay. It was only a couple of decades ago that openly gay people started rearing children. He calls himself, and other gay children of gay parents, ''the second generation.'' For him, the experience has brought ''an appreciation for things that aren't necessarily the norm.'' For others, it has not always been so easy. ''We're not your average TV family,'' Mr. Cherubin, a music librarian who lives in the East Village, is the gay son of a lesbian who lives with her companion, and he belongs to an unusual - and, for some, unsettling - kind of family. Though his father died five years ago, he still has his mother, and his other mother. DAN CHERUBIN considers himself pretty lucky.