How to argue for the acceptance of homosexuality from a biblical standpoint?

I recently got into a debate with someone over this. I tried telling the person the bible bans all sorts of things besides homosexuality. what he told me is that the old testiment doesn't count because God released Christians from that covenant and only the new testemant applies. I told him the nt also bans divorce and condones slavery, but he just told me he doesn't like divorce anyway and that slavery in the bible wasn't "really slavery" because they were just servants.What is a simple religious argument I could use to support gay people that a christian could just not simply refute? I know you shouldn't get into it but I want something quick and easy to say when people wanna say "gays are sinners."