Most Helpful Guy
Sounds like what you need most here, is perspective. Whoever told you that the core reason relationships exist is purely for recreational sex, lied to you. Let's get that out of the way right now.
I'm in an LDR with a gal that has little reason to stay with me, and I have little reason to stay with her. We're an ocean apart, and you can only imagine the temptation that brings. But the commitment, in some ways, is its own reward.
Is your man:
1. Bringing you closer to God?
2. Someone you would count on to pick up your medical bills if you have to not have a job for a while?
3. Determined to have you legally be a part of his family?
4. The kind who'd stay faithful to you, even if you killed someone in a freak accident and face manslaughter charges?
5. The type to bring moral and emotional support, even when you face your greatest fears?
6. Willing to do all this for you regardless of how or how soon you are able to reward him?
Are you wholeheartedly convinced you could be all that back for him?
If a shortcut to pleasure that you're not even entitled to (1 Cor. 6) is the only reason you can think of for being in a relationship with him in the first place, then you are in a relationship for the wrong reasons to begin with. As such, it will never bring you what you truly seek: fulfillment. The most you can do is get a few cheap thrills; but orgasms only last a few seconds and are gone. And if there is nothing behind them, they betray you to a horrible emptiness.
Others on this site will scream bloody murder that I'm telling you this; but it's true no matter how much they shake their fists at it.
Your compulsion toward infidelity is because you desire fulfillment, of a sort you will never get from sex. But that's all anyone bothered to tell you. You sense that your man may be able to please you bed; but that there's nothing behind the sex machine and the curtain.
You're hoping cheap thrills with another will fill that void. But if you seek to fill the void that way, you will make a fool of yourself. You'll only find more emptiness down that path. And more violations that you'll have to make excuses for. Excuses that will also never make you happy.
We can PM about it some more if you're interested. First thing I would suggest you do, is ask him what future he sees for his family through you. Then ask about contingencies...