Basically, yes.
So getting MARRIED ought to count, no? So why should ADULTERY be legal? Absent 'shotgun' weddings, aren't married couples in the same place (using your odd logic) as volunteers?
I don't really think it is appropriate for the state to get involved in marriages. But, let's assume that it is involved in certain cases. For example, people who are not religious and want a judge to preside over their wedding and make their commitment in the form of a contract. In a case like this, where sexual faithfulness is part of their commitment to each other, I would support punishing adultery on the grounds of breach of contract.
But to extend such a punishment to all marriages, regardless of state involvement or the terms of the commitment, I would say no.
And how about draftees? Or 'volunteers' whose term is 'extended'? Ought they get a "pass" from the UCMJ laws regards sex?
I don't believe in the draft either. Draftees shouldn't fall under the UCMJ. If a volunteer knows that involuntary extension is a possibility at the time of his enlistment, they should not get a pass. They are still volunteers in that situation.
I should probably add that most of the provisions of the UCMJ discussed on this thread make little sense to me, and I think they should be done away with. They are rarely enforced, and I don't see any need for them.