For me, I've thought about cheating before. Many many times. Say whatever you want, but it's in humans nature to get attracted to someone good looking and wonder what it's like to kiss her/him, you get my point. Whether you choose to act on that temptation is an entire separate issue.
1. The feeling of guilt and miserableness that follows
I'm someone which feels guilty if I did something that I know would hurt someone I like alot.
I believe it's the same for almost everyone, if you really liked someone, and by doing something you know that would really hurt him, the feeling of guilt is intense.
I would feel especially upset if my boyfriend has been treating me really well and is unsuspecting of me.
Having to bear the heavy burden of not knowing when your fling would be exposed is just not worth it. And eventually, if you choose to tell him of that one night stand you had, even if he forgives you, it would be quite impossible to have his trust in you fully rebuilt for a long while.
2. The other guy
If there was any brief attraction or feelings between you and the other guy, I think it would be gone the instance you sleep with him.
The other guy probably thinks you're hot, knows you have a boyfriend and desperately wants you. If you do like him, end things with the boyfriend first before taking things to another level with the other guy.
If you decide you want both guys, the other guy would probably treat you as a cheap fling, after all if you can sleep with him while you're in a relationship, you would likely do that to him too and he would have no interest for a girl that is so easily attainable.
And I think it's really dumb to give up a great relationship (that is if everything in your relationship is going well), a boyfriend who genuinely cares about your happiness and well being, who truly loves you for all your imperfections and little embarrassing things that you try to hide, for a guy who barely knows anything about you and just wants to get you to bed.
3. The embarrassment you would bring to your boyfriend and yourself
Presumably, if the other guy is like most guys, he would likely go around telling his friends what a slut you are, and boast about how he got you into bed.
The mistake most of us females make is trusting guys too much. Yes, sure he promised to keep it to himself. This means telling 10 of his close buddies, and gossip like this spreads quickly. My problem is that I mix with guys too often, and I hear things like this all the time which makes me extremely apprehensive and cautious of anything that has a penis. (lol)
Imagine how your boyfriend, the one who gave you sweet kisses on your forehead, the one who planned for weeks how to give you an amazing anniversary dinner, the one who thinks about you every night before he sleeps and smiles at your text messages, yes, the sweet loving boyfriend, imagine how he would react when he heard of how his girlfriend got "fucked" and how easy she is.
Would that few minutes of pleasure make up for how crushed and broken he would feel when he finds out? How embarrassed he would feel, how sad he would be?
I really can't think how it'll be worth it.
4. You could get STD
This is probably horrifying and the worse punishment ever.
I've got a girlfriend who cheated on her boyfriend, and she got gonorrhea. She told us that it was the most painful experience ever and she had to lie to her boyfriend that she was on her period when he wanted sex.
It is definitely safer to stick to sleeping with a guy you're in a relationship with, than to sleep with someone you barely know and risk contracting a sexually transmitted disease.
5.You gave him a trump card
Your boyfriend, who is in love with you, decides to forgive you. You are elated and extremely relieved, and decide to never cheat on him again.
Unfortunately, it's not over just like that. He will use the incident of you cheating on him to justify his paranoidism if he calls you 30 times when you are out with your girlfriends. If you catch him looking at a hot girl, or texting a girl, he'd remind you that you cheated on him before and he forgave you, and his nowhere near cheating on you so how dare you even accuse him of anything.
He can say his forgiven you entirely, but put yourself in his position, would you be able to entirely forgive and wipe out the incident fully from memory? I think not. There would always be a certain degree of doubt and this could be the cause of many future problems.
6. He will need time to get over it
And the uncertainty you have to go through during this period is just really depressing.
Getting cheated on is not something anyone can come to reality with in minutes. He would need time to think if he loves you enough to be able to continue the relationship knowing you've slept with someone else. He would consider if he should give the girl who has been liking him for the longest time ever a chance instead of rebuilding the tainted relationship with you, the cheating partner. He will probably date other girls and if he find happiness with them, there is absolutely hardly any chance or reason he will want to get back with you.
If you still love him at this point of time, you're going to be depressed, upset, you're going to self-blame, self-reproach, and hate yourself tremendously. If he was the one who cheated on you, at least you know you did nothing wrong.
But knowing you're the cause of the entire great relationship crumbling, you will hate yourself for that.
If you're still thinking of cheating on your boyfriend because the sex life isn't that good anymore, see my last reason - the one I adhere to and remind myself everytime when I get into a relationship:
7. You love him
And if you truly love someone, there is absolutely no justifiable reason for wanting to do something that would bring him tremendous pain and sadness.