Well to make a distinction: women should occasionally have multiple orgasms over the course of one sex event, if they are in a stimulating-enough situation. A game such as NewLife, although it is slow in development, actually models this well.
For men, they should usually have 1 orgasm. Then there is a refractory period. However, men should not stop having sex until they orgasm. It is not only because a man would not do this, but also because of something called epididymal hypertension, or blue balls. I don't want that to be modeled in game--easiest route is to just end sex once the guy orgasms and not before.
The multiple orgasms in group sex in the game are sort of cool, don't want to fix that really. Multiple male orgasms are possible, but an insight or special situation like this should be required to make that happen in-game.