It was destructive, he knew, but still he came back for more. They were both addicted to the rush of endorphins; it certainly wasn't love that drew them together. This relentless push and pull - always fighting to gain the upper hand - couldn't be mistaken for romance. It was violent and hateful and everything you shouldn't want from another person.
He wasn't sure what had drawn him to the older boy - it certainly wasn't his sallow appearance or superior demeanour, and his personality was far from appealing - and he couldn't say what the other boy was getting from this, either. Maybe it was simply the satisfaction of shagging his tormentors little brother. If that were the case, though, Regulus would tell him not to bother; Sirius wouldn't care either way.
And he certainly couldn't explain why, when the older boy graduated, he missed him.