<human>: Can you provide a brief rundown of the text in within 3 sentences?

First time using reddit, so apologies if I've missed anything out or if the formatting goes weird - am on mobile.

I have a really silly problem but it's driving me mad. I have a wonderful partner with whom I have been for over 4 years. We've lived together for most of this time and before that, I was quite... promiscuous, so I'm aware of what I'm 'missing out' on, which is nothing. We get on extremely well (though like everyone, we have small squabbles) and he treats me with love and respect. I honestly cannot praise him enough; he's everything I've ever dreamt of, and more. He is also very happy with me and we have a strong future planned together.

My problem is, every once in a while, I get strong urges to kiss other people and fantasise about fucking them. They feel so vivid that I have to hold myself back from carrying them out. I haven't done it yet - the one thing which is holding me back is that I know it would break his heart which in turn would break mine -- the thought of hurting him fills me with horror. But how can I deal with these thoughts? I find it very easy to disassociate sex and kissing from relationships and I'm genuinely concerned that i will end up cheating on him. 

Sorry that this is all over the place, I'm just distraught at the thought of hurting him.  The types of men I have these thoughts about tend to be emotionally very immature and are very aware of their own attractiveness.
<bot>: how to fight urges to cheat on blameless, lovely boyfriend? (Also, I know this doesn't make me look good, but it's a human problem in an imperfect world --- and I haven't done anything yet!)