This happens to be a subject I am very intimate with. I myself have never cheated on the person I was with at the time, and by cheating I mean touching, kissing, screwing, just a bit of the dry hump and emotions being involved.
My ex-husband however did cheat on me. It's not just sex with another person, if it was just that I could have handled it - I wouldn't have liked it, and I sure as hell would have gotten out of the relationship as soon as I found out, but it wouldn't have devastated me to the point where I wouldn't let anyone into my life for 11 years because I was afraid someone would take me down that twisted ugly low self-esteem road again.
Cheating to me is taking what one person gives you, trust, love, their body, and even their lives in your hands and then fucking them over by having a relationship with someone else. By relationship I mean more than a one night stand, dating, seeing someone on a regular basis just to be with them, telling them you love them etc.
My ex had many relationships like that at the same time. He dated (at last count because they keep coming up to bite me in the ass) 4 strippers, 1 delivery driver (female), 1 judge who shall remain nameless so I don't get my butt thrown in jail (male), my cousin (female), and my best male friend who just so happened to be an exotic dancer at one of the local gay clubs.
The bisexual aspect has nothing to do with it. As far as I'm concerned most people have a bit of that in them to begin with. I see nothing wrong with it (and if you've read any of my works you'd know that). I see the cheating as the problem - not the sex of the people he chose to do it with.
I'm a bit more open than most when it comes to this as I can forgive a lot (and I did - many times to the point where I forgave him his first 3 before I got my head on my shoulders and realized that everything he was doing meant that he didn't love me - he loved controlling me)- although I never forget. Basically what I'm saying is this - if you are in a relationship and you find your fingers wandering, your affections going to someone other than your significant other you need to stop and take a long look at yourself in the mirror. Find out if this is something you really want to do. Can you handle losing everything you have for a fling - because if the answer is yes get the hell out of the relationship before you do anything about it. Get out before you hurt that person more than can be healed.