Here's what I wrote on the same topic in another thread. The short answer is, if YOU have a problem with it and YOUR conscience would make you feel guilty, don't do it. If the other dude is a friend of yours or a family member, don't do it.
But,... if you don't give a fuck about the other dude and your conscience is clear, go right ahead and here's why:
Quote:
1) Monkey branch theory. Women are always looking for an upgrade on their current boyfriend. Now before you panic, she will stay with her current dude unless the new guy is better in most departments. So she will cling to the current branch until she finds a new branch which is solid enough to swing towards. If she tries the new branch and it's not as solid as she thought, she will swing back to the old one. Solution: Take a note out of a girl's book and monkey branch yourself. No matter how hot your GF is, there is a hotter cooler girl ready to fuck you. This attitude also keeps your girl's interest level high, because she knows you could trade up at any time, so she works harder to keep you, she doesn't get fat, she drops fat cash on beauty SPAM, sucks your cock constantly, makes nice food for you etc.
2) Single girls are often single for a reason. The hottest girls are often taken.
3) Girls in high demand often say they have a boyfriend as a shit-test. If you give up at the first hurdle, you could be missing out on hot single pussy.
4) With every girl with a boyfriend I fuck, I learn all the wide-ranging reasons why their boyfriends suck and fail at relationships and how to avoid those mistakes myself.
5) I'm slightly ashamed to admit that I love showing up to a club and being "that guy" girls whisper and gossip about. Ashamed because i don't like getting attached to my ego, i certainly don't show it, which is the main thing. Once you get a reputation as a good fuck who can make a girl cum hard and fuck all night, you get passed around social circles like a piece of meat. The rumours help me tap into more untapped pussy because of my reputation for being discreet and respectful of relationships.
So to sum up, if you wanna be a muthafukkin' pimp playa, then tap it. If you're looking for a monogamous relationship or a girlfriend, stay away and find a single girl.