I think we all need to accept the fact that during low points in anyone's lives, their resolve to uphold their beliefs and ideals weakens, and there's a chance that your significant other may cheat on you. I don't care what anyone says, everyone has their times of weakness. At one point you're in a happy relationship with someone, then you're fighting, and all of a sudden one of your best friends becomes a very tempting alternative... until you come to your senses, which by that time it's usually too late.
Limiting who your significant other can be friends with? Are you joking? That's like making abortions illegal, "That will stop them." No, it won't. Best you be an open couple and fully understand the situation rather than put your relationship in a position where you have to be secretive to have a friend.
I don't think it's worth the energy to worry about something you could never really stop anyways, unless you lock the other person up, which is ridiculous.
