I am not a big religious person so I can't speak on it extensively, but I do believe it's a sin to step out on one's wife (assuming your talking about cheating within marriage).
You are correct Fresh, from a Christian religious perspective 'thou shalt not commit adultery' is #7 on the list of the Big Ten, unless you are Catholic in which case it's #6. Condemnation of adultery is not solely a Christianity based practice though; Islam, Hinduism, Buddhism and Judaism also discourage adultery. Basically any culture that holds marriage as a sacred act, and believes monogamy is the ideal standard, will likely consider adultery a sin... which covers pretty much every major world religion except Satanism (if you happen to consider that a religion). Religious viewpoints on the subject have historically centered around the man’s rights with regard to an adulterous wife, however that street runs both ways. I’m not a fan of double-standards in anything.
While I personally believe it to be morally wrong, that's based upon not only my religious upbringing, but also on the fact that as several other's have stated, it's a betrayal of trust. Whether the other party is aware or not, it's a betrayal. I don’t like to use the term entitled, but if I were in a committed relationship I do think that who else he’s screwing is information I should have, and that goes both ways. Not to mention, there is likely something seriously amiss in the relationship if one party begins questioning the morality of cheating, merely asking the question of yourself suggests you know it’s not right. After all if you truly don’t believe it to be morally wrong, why would you waste time debating the matter in the first place?
Are there people who would prefer not to know, sure there are. Some people prefer to live in blissful ignorance, rather than accept the cold, hard truth; while others don't care how bad it is, they just want you to be honest with them. Red pill vs. blue pill. There will always be people who prefer to believe the world is all sunshine, roses, and kitty cats, while others will prefer reality no matter the cost. Since your question seems to center primarily around whether it's morally wrong only if the other party finds out, perhaps the question you should ask of people is whether they would want to know or not? As for me, I’d want to know, and chances are I would find out at some point and in that case sooner is most definitely better than later. The longer I spent unaware, the greater the reaction when I did find out. Best to get it out of the way early on.
