Why is it more wrong for a person to cheat than it is for one partner to completely deny the other physical companionship? If sex is sooooooo important then the person not giving it up needs to understand that their partner needs and give it up occasionally.

I get that you feel sex is important and I happen to agree with you. I agree that there are consquences and dangers. You are ignoring the other side of it though in this case. That being what I just said above, if it's important, then it's always important and you can't expect your partner to happily live without it just because for what ever reason one partner doesn't feel like having sex.