Why extra marital sex is wrong?

If you and your spouse have no problem with an open relationship so be it. I don't think its wrong it just comes down to who you are as a couple. Me personally I couldn't do it. Just the thought of my husband doing something with another person makes me upset and the thought of myself doing it makes me feel disgusted. Its not that I think he would leave me or that I would leave him I just feel like he's mine and I'm his only, no one elses. My husband is the only one I want to to be with, I don't want anyone else but that is how we are. That is OUR relationship I would never impose my lifestyle on someone else there is no need.