It's possible to cum multiple times as a PC with a cock. But NPCs with cocks won't. The scene also ends as soon as everyone has cum at least once. Even as a PC with a cock you probably will only cum once, unless you have multiple partners, or you are the only one getting pleasured somehow -- generally a bug in the algorithm though, which does happen occasionally when you are "not in control" and your partner is simply refusing to let you pleasure them.