Silly Pumpkin Eaters

Relationship-wise, cheating is not cool--in fact it's stupid. If you're cheating on a girl/boyfriend or spouse, you don't really love them or your relationship arrangement isn't what's best for either of you. As usual "in my personal opinion", there are two possible reasons for cheating:
1) There is something wrong with your partner
2) There is something wrong with you

Either way, the relationship is not somewhere you want to be.

Reason #1: Perhaps you are cheating because there are certain things that your partner cannot provide for you, and you look elsewhere for these things. Clearly, if this is the situation, why stay with them at all? By cheating on them and avoiding the problem, you're going to be stuck with a lousy partner. If/when they ever find out about your affairs, you'll be obliged to make it up to them or the take the blame for the whole thing (that is, if they don't dump you and everyone gets to know you as the cheating jerk/hoe). Even if they don't dump you and they forgive you, who knows when you'll ever get away from them at that point--congratulations, you now owe them your life, so to speak. Alternatively, in the case that they never figure out you were cheating, you'll carry that guilt with you and so will your cheating partner. Why do that to yourself?

Reason #2: If you cheat on your significant other, there are a number of things that could be wrong with you.
-Commitment Issues: If this is you, why do you need to be in a relationship in the first place? Why rein yourself in with a nagging woman or a jealous man--go out and have fun; enjoy the single life like you were born to! If it suits your fancy, get involved in a number of random flings, live double, triple, quadruple lives (they'll never know, as long as you don't forget their names).
-Insecurity: Sometimes people cheat because they're developing a "backup" or they want to see who else wants them--don't ask me I don't come up with these crazy rationales. If you are insecure about your relationship with your significant other and you really want it to work out, the best way to resolve this is to talk to them. Being in a relationship or marriage comes with a certain responsibility that you discuss problems or concerns that either of you may have and address them accordingly. If you're meant to, you will be able to work this out legitimately and with the least amount of bloodshed.
-Lack of Interest: Similar to Reason #1, if you're drawn to someone more interesting, you don't truly love your current partner. Break up cleanly and do things right.

To sum things up, I feel that if you have a truly "justifiable" reason for cheating, you shouldn't be in the relationship in the first place. End things right and be humane--there's no need to put your partner or yourself through the drama of a spoiled affair.