So, the three of you are having a great time. There is sexual tension, tantalizing exploration, heavy breathing, licking and delicious grinding everywhere. Then a big grunt as your man can’t contain it anymore and comes waaayyy too early…
See the bigger picture
There are a few things you can do, some better than others (to say the least).
– Call it a night. Not a good idea because the frustration of all three parties involved could take a long time to repair
– Ask your guy to watch, go for a walk or be a limited participant while you and the other girl continue the fun, hoping he’ll get excited again soon enough to be useful. This could end in a downer if he doesn’t rise to the occasion again, which is always a possibility when a guy feels obligated to get a hard on rather than just getting one in the natural flow of things. So if you choose this course of action, don’t bring up his lack of control, and give him some time (and a good show) before you start fluffing him up again
– Agree to resume after at least an hour to give him time to recover. Watch a movie, grab a bite, do whatever, but try not to bring up what happened, especially not in a joking matter! The way guys are wired is that if they feel they have to get it up, they often can’t. Focusing on something non-sex related for a while is the best way to ensure a solid recovery. Once you’re back in the flow of things, make sure he gets enough attention to stay hard while not exciting him so much the same thing happens too early again.
It sucks that each of these will take some of your fun away because monitoring the situation will cause you to be less carelessly uninhibited, but keep in mind it’s a lot better than a ruined evening that will haunt you guys for a while, and that he’ll likely be a lot more mindful of that not happening again in the future, whether alone with you or with an added party.