why cant my boifriend nutt?

There could be a lot of reasons why your "boyfriend can't nutt".

I assume you mean, why can't he climax or have an orgasm.

There are many possibilities. You suggest that maybe he's having sex with someone else. Maybe, but why would you think that? There must be some underlying trust issues for you to go there so quickly.

Sometimes, stress can put a guy off his game.

Or the fear of discovery. Are you in a private place? Free from distractions?
Is there a risk of getting caught?

How about fear of pregnancy? Are you two protected?

Perhaps he's been playing sports hard. Could he be tuckered out?

And what about the possibility that he's just recently masturbated? It take a certain amount of time for a man to recover after coming.

How frequently are you having sex? Perhaps it's too frequent for him to recover.

It sounds like perhaps you want him to last longer than 4 minutes.

Many men can only go for 5-6 minutes...before coming, but it sounds like you're saying he's uninterested, or unable to go on. That might be an erection issue.

I think the next step here is for you to discuss this with your bf. Have you asked him what's up?

The two of you need to talk.

Men are mental too. Sometimes I can't "nut" Sometimes when I am very excited and having a long session I can't seem to ejaculate. It gets more painful with time and an erection makes it worse. It happens sometimes.

Also, some men can ejaculate multiple times with practice and not go soft in between. This goes away as you age and sessions get fewer and farther between. I can control my ejaculations to a large extent but I have no control when they just won't fire off. It just happens.

Like sometimes my nuts hang loose and slap and sometimes they tighten and go up inside and won't drop until after I ejaculate. I would not speculate performance on fidelity. It's too easy to make a huge mistake this way. 

TALK TO YOUR BF ABOUT SHARED COMMITMENT. It's the best way for you to handle things until you have proof of infidel. We are not married is no excuse. Unfaithful single is unfaithful married, period, end of discussion. Unfaithfulness is a state of being and not a legal definition of a relationship

I think that your boy friend is having sex with some one else becuse the cum doesnt come out easilyafter a longperiod if you have sex alot with your boyfriend then thats can be theproblem yous hould avoid sex for may be 1 or 2 weeks or even 1 month so that is health will increase and he can have sex with you ffor about half an hour  if he f**ks you harder and faster for a short period than it can be a problem ......o.O