Prostitution.
First, I'm going to assume most spouses aren't cool with their spouse getting it elsewhere. I'm also going to assume that more than one of us would nail a prostitute when the wife was being bitchy if it was legal. Assuming you still consider it victimless(you want some, the wife won't give it up, the prostitute will for a fee), you're still going to have some broken homes. Wife finds out, takes the kids, kids have no father figure, kids get a little unruly, etc, etc, etc.
Second, there's the mental aspect of being a whore. Young women(and men because you know the gays need equal rights) make bad decisions. It's part of the growing up process. Are you willing to say that no one will look back on their youth and say "I can't believe I let a fatass like swampy flop around on me for 3 minutes three times a week for 3 years" without regret? I'm not sure you can come back from that.
Third, it's my contention that men get involve in relationships for the physical aspect. We've got certain needs that have to be taken care of. Unfortunately for us straight guys, there isn't a vast expanse of women willing to handle these needs unless they're getting something for it. Since prostitution is illegal, we have to date them and maybe even marry the one we find tolerable to satisfy our carnal needs. Making it legal to buy the poon would, without question, stop many men from getting involved in deeper relationships.
Fourth, if the government run sex houses are charging $50, do you really believe that some enterprising gentleman won't get himself a stable of whores and charge $40? You don't eliminate prostitition, you just drive it further underground(use abortion as a model).
In summation, legalized prostitution will wreck family values. And damn sure won't eliminate the illegal sex trade.
Next.