We grow up in a world where the idea that if you're in a relationship then you don't sleep around is pretty strong. But a lot of gay guys don't follow that pattern and still have strong happy loving relationships.
It's kind of ordinary to hear that two men who love each other, maybe even have had a Civil Union, still have sex with other men, sometimes outside the relationship, sometimes together with their partner. Some guys say that they just want emotional monogamy - fuck who you like but only love me.
It's not for everyone though, and it can definitely raise problems. The gay world is full of easy sex if you want it, and that can be very tempting. We're also seen by the world as being sexually different before anything else. So that focus makes sex play a bigger role in who we are than it does for many straights.
Even when you've agreed to be open it's easy to get jealous, that's for sure. A lot of us seem to manage if it's some anonymous fuck your lover never sees again, but not if it's someone who becomes a regular fuck-buddy. If you're seeing the same guy again and again for extra fun it can be threatening to your partner and make him question how committed you are to him.
Other guys are very happy being monogamous, it's all they want or need. Some guys begin relationships that way then change as they grow and mature over the years - there's no one right or wrong way. Sometimes you still love him more than anything in the world, but you've both lost the lust for each other you used to have. It does take really good clear communication to make it work though, that means being able to talk about how we feel, and that can be a difficult thing in any situation. And some couples are perfectly happy fucking only with each other for the rest of their lives, and that's just fine.
The trouble seems to happen when the communication breaks down or one guy lies. If you think you're only fucking with each other then find out he's been having fun on the side, well, it is pretty devastating. It opens up a whole can of nasty for everyone. Lies in a relationship can really damage the whole thing, are hard to recover from and sometimes destroy it. While it's pretty hard to sit down with your lover and go "Babe, I know we promised not to sleep around but last week when you were visiting your folks I slipped up" it's still better than him finding out some other way.
And one way you find out is getting an STI from your boyfriend - and in some cases getting HIV if he's been having unsafe sex with his fucks. If we keep up the habit of using condoms even in our relationships it makes the chance of anything like this happening much less - if you are automatically used to reaching for the rubber with the man you love then chances are you'll keep the habit up with someone else. And doing that is much easier than turning to him and going "You know what babe, I think we should go back to using condoms between us." Staying in the habit of wearing them every time is about looking after the man you love, the man you fuck, and yourself.
You can make open relationships work though - heaps of couples do, but they usually have very clear rules in place and stick to them. Agreeing to always use condoms when fucking is automatic. This doesn't just protect the guy wearing the condom, it protects his lover as well. Agreeing that either one is able to say you're uncomfortable with the relationship being open and want to make it exclusive again is pretty common too. The trouble is of course, generally we're not that great at talking about sex in a calm and comfortable way, so even if you agree on the rules, bringing the subject up can still be difficult.
Like most things in a relationship, if you want an open relationship it takes work, it takes time, patience and really good honest communication with each other.