Sebastian isn't gay. Even though he dresses and bathe Ciel doesn't mean he's gay. That's what he's supposed to do. Ciel made a contract with Sebastian. Sebastian is his butler until Sebastian swallows his soul.
On the other hand if Sebastian was gay he would be in 爱情 with Grell. (Grell is a dude.) Grell and Sebastian would be a couple, but their not because Sebastian isn't gay.
Note to Sebastian Haters: Sebastian isn't gay. He is too cute for an 日本动漫 character. And if 你 still say that he's gay; go suck a d**k. (I'm so sorry for my language. I'm really angry right now.)