Thou shalt not commit adultery, Exodus 20:14
This happens to be one of the few commandments I generally agree with. I do have a couple of small disagreements though. My first problems comes from what was generally considered adultery, until modern times the only person who would be blamed for breaking this law was generally the woman involved. Men were mostly overlooked.
The other issue I have is that I think this is primarily an issue of honesty. If people agree to some form of poly-amorous relationship, as long as all parties agree and no one is being dishonest or forced into anything against their will I don't really see the problem with it, though technically such behaviors would run afoul of this command.
In any case, I can't think it is good or reasonable to criminalize sexual infidelity on its own. This seems to be something those in sexual relationships need to work out on their own without interference from the government.
This commandment, at one time, had a larger place in the legal system in the U.S. In fact are still states that carry a law punishing adultery varying from a life imprisonment in Michagan, to a 10 dollar fine in Maryland. Of course these laws are very rarely used, and several supreme court decisions have made most states reluctant to attempt to prosecute people under these laws, since they are often overturned in higher courts.
Though the government has been rather inconsistent when it comes to laws against sexual behavior I generally find that making sexual behavior illegal between consenting adults to be something unneeded and counter to the constitutional guarantee of equal rights.