You make them like it, they cum, and you don't cum

As you fuck them, the crowd makes oooo and aaaah noises. The girls get turned on by the exhibitionism, moaning louder for everyone's benefit. They pinch each others nipples as you really hammer their tight pussies, first your girlfriend, then her sister. Back and forth, you switch from one to the other. The crowd is cheering you on.

Your girlfriend shouts, "Oh fuck! I'm coming! Aaaaaah!" You feel her cunt squeeze you, but you maintain control. Her body jerks and shakes as her voice rises in ecstasy.

Once your girlfriend comes down from her climax, you switch to her sister, slipping inside her and really rogering her. The crowd is cheering loudly. Then she shrieks wordlessly as she comes, her pussy fluttering around your cock. You manage to hold off on coming. The assembled masses clap in appreciation.

The two sisters roll over, both looking lustfully at your stiff cock.

(Mark +2 xp for making them cum)