Draco is having regular casual sex with one of the Weasley twins. It started after a Quidditch match, where they started trading insults, then blows, and finally kisses. And it's just carried on from there.
He hasn't actually worked out which twin it is yet, and he hardly likes to ask. He imagines that if the guy didn't like being called "Weasley" during sex, he'd say, "Call me George (or Fred)", and then Draco would know. But he hasn't, so it must be all right.
It hasn't occurred to Draco that the twins have been changing places and sharing him.