I agree with Sam. I'd never leave the bedroom if I didn't get some of this stuff out in other ways! That was supposed to sound like a bad thing...
Anyway, as someone who has cheated (not on my husband, but way back when I was young and stupid) and who has been cheated on, it's just not acceptable in an exclusive relationship. Trust is one of those things that you need in a good relationship and if you are cheating (I'm speaking in the physical ways and in my mind, anything from making-out up is cheating) you're breachinig that trust, and that doesn't feel good from either side (for most people anyway).
Besides, cheating isn't worth all the extra work! You've got to remember all the bullshit you had to come up with about where you were while you were banging someone else, and while you're with that someone else, you have to make sure you don't get caught. It's exhausting really. And there's the emotional exhaustion too, mountains of guilt (if you have a conscience) and the effort of two relationships, one of which (at least) you are always on the verge of loosing.
Why anyone would put themselves and someone they profess to love through all that, I'll never understand. Luckily in my case, the guy I was cheating on was cheating on me, so we both got out in fairly good shape when the shit hit the fan... but I tell you, never again!