In my early 20s I too had a very black-and-white view of cheating. I simply cast off the men as losers with no self-control, and the women as lying sluts. Now having been cheated on, and subsequently cheating back, I have actually softened quite a bit on the issue. While I don't necessarily excuse the behavior, I'm nowhere near as quick to place moral judgment upon the cheater, because frankly, I think more people than we are willing to give credit to are driven to cheating by a combination of lack of communication and an inability, often outright unwillingness by their partner to hold up their end when it comes to intimacy. It's similar to how we see theft and homicide as objectively bad choices that people make, but will overlook the starving person that steals food, or the combatant that defends well being to the extreme.
Here's my take: because the preceding posts are really par for the course as far as opinions on cheaters/cheating, i think because the posters themselves can't imagine themselves exhibiting behaviors and withholding affections that motivate many to cheat, they have such an negative reaction to it...but as I said before I think cheatings "wrong" can be filed under the same "wrong" as theft, homicide, or even cannibalism; things none of us would intend to do under normal circumstances, but things we can be pushed to and have to reconcile within ourselves after the fact.