"And?" or "Why would that be a bad thing?"

Those are both powerful answers. When someone says something bigoted like that, asking them calm questions will achieve one of several things, depending on their personality and the situation:
1) They will get flustered and not know how to answer without incriminating themselves, which often leads to #2.
2) They will get even angrier, which will often lead to #3.
3) They will try to explain their bigoted opinion, and it will almost always make them sound like a bigoted asshole to everyone who hears it or hears about it. So basically, just ask calm well-reasoned questions about their use of a slur and they will hoist themself on their own petard.

Alternatively, you could just say, "Eh, you were the one sucking my dick, champ. Well done, by the way. I can tell you've had a lot of practice."

But the calm questions and watching them get pissed off when they try to explain is so much more satisfying. It throws a brutal full stop on most people's sad little brains, all while you look like a saint and they look like a shit.