Hi lelola - I used to live in Brighton and knew a lot of gay parents who were members of a local organisation called Rainbow Families. They spoke very highly of it and seemed to make a lot of social connections through it.
I'm not gay myself, but the couples I knew used the playgroups around Kemp Town/Queens Park/Lewes Road/Fiveways without any incident. Gay families are pretty common in Brighton!
I'm now in Sydney but when I lived in Brighton most of the gay parents I knew sent their kids to Queen's Park primary (very small school though so you now need to live practically on the doorstep!). One of the nice things about Brighton is the fact that most people you come across when having/raising kids won't have the "family = mummy, daddy and baby" thing going on.