Not so fast.....
Adultery is an actual problem that affects good order and discipline. People who commit this offense have no honor. They are, by definition, liars and untrustworthy. If your own spouse can't trust you, then why should anybody else? I know that in normal society that adultery is considered "no big deal", but that's where it's parallels to homsexual conduct stop in terms of military service. Adulterers conduct can greatly affect the effectiveness of a unit, particularly on the small unit level. I don't believe that homosexuals lack honor simply for being homosexual.
I have seen quite a bit of adultery get punished under the UCMJ. Some of it as a single offense (not piled on with other charges). There are varying degrees of this offense, and it probably gets brushed under the rug more than it should, but that's probably more because of the burden of proof than simple tolerance. If you flagrantly commit adultery, you should (and probably will) get hammered for it. It shows that you have no regard for your oath to your spouse (and God, if you see it that way), your obligation to obey regulations, or your own integrity. You made a choice to dishonor yourself. This is not about the military being stuck in the past. There are real consequences to the command for your conduct.
If you want to elimninate a meaningless and outdated part of 134, it should be cohabitation. I've never seen that affect good order and disciplne, nor have I ever seen it enforced.
As for adultery rising to the level of conduct unbecoming, article 133 only applies to officers.