Regulus had no idea what to do when someone unexpectedly grabbed him from behind. For the briefest of seconds, he thought it might have been James, because that was exactly the thing that James liked to do. However, the moment that the stranger spoke, he knew, beyond any doubt, that it wasn't James. His voice was perhaps a bit lower and James didn't laugh like that - and James (while they might have had sex on the beach) wouldn't be stupid enough to grab him where people might see.
Regulus froze up, because this sort of thing just didn't happen to him. His voice seemed to have died completely and his brain had short circuited. When the man's hand found its way down in between his leg, Regulus let out some strangled little sound that would have been completely embarrassing if Regulus hadn't been trying to figure out how to get out of this situation.
"I don't think I'm who you're looking for," Regulus managed to say, his words coming out fast and a little high-pitched. He could practically feel the other breathing down his neck, which was a little disconcerting in and of itself.