So I'm posting this on my "not me" account, and this is kind of long, but here was my experience with my last GF.
You know how somehow every girl is interested in you when you already have a girlfriend? Well it starts like that. At first you're not going to cheat, but you want to see if you've still got it. Now you know you've still got it and just want to dance with her a little bit. All of a sudden you're dancing and making out, but you're not sleeping with her so its not cheating yet. Then one night you're really drunk and had a fight with your girlfriend, and you end up doing the same thing and boom you've slept with this girl and are officially cheating. Now you keep sleeping with this one girl because: "Hey, nothing bad happened the first time, this girl is just a physical outlet for me and I still love my GF." Well time goes on and you've fallen so far over the line that you can't even remember when you really crossed it, and this new girl wants more from you, and your GF is wondering why you suddenly have to work late but are making no more money, and you've got to try and find a way to break it off with the new girl, but it's hard because you kind of like her too. Suddenly you've constructed a web of lies so complicated that you're not even sure you can unravel it, and you're not sleeping well because if your current GF finds out the shit will hit the fan, and if your newer girl finds out, your GF will eventually find out. You get into this situation where it's just easier to put it off, and it's sort of like when you put off breaking up with someone but to a greater extent because the disincentives are so copious.
TL;DR You push the envelope, little by little, and by the time you've realized you've crossed the line, everything is fucked up. Now you've got to try and break it off amicably, but it's not that easy because you like both girls and they both like you, so you try and let it sit, but it will, more likely than not, blow up in your face.