If it's an unconstitutional law, then no, I don't particularly support the government enforcing that, and I think the executive should be smart enough to decline to defend unconstitutional laws so the courts can overturn them more quickly - see e.g. https://www.justice.gov/opa/pr/statement-attorney-general-li... about DOMA section 3 in particular. (That said, as it happens, I am supportive of the end result of Hodges trying to defend his state's law all the way to the Supreme Court, because it caused all such laws nationwide, including all of DOMA, to be declared unconstitutional.)

If it's an "immoral" but constitutional law, then I'm in a harder place. As a leftist Christian I'm pretty aware that my personal views on morality are not particularly widely shared by my fellow citizens (even though I happen to agree about DOMA being immoral), and part of the deal of living in a constitutional democracy (as opposed to an absolute monarchy or a theocracy) is that I don't get to impose the entirety of my morality on others and they don't get to impose theirs on me. And that's a great deal for me, knowing just how many Christians think merely being left-leaning is immoral and just how many left-leaning folks think merely being Christian is immoral. Our society has decided to enshrine a subset of widely-agreed morality into the Constitution - that certain laws are so unjust that they may not be made - and then we agree that the rest of it is up to the mechanisms of democracy.

So if it's constitutional, I'll certainly do my best to argue that the law should be repealed, through the legislative process, and I'll try to convince others to convince their representatives in turn to repeal the law.

But if it's constitutional, and it's popular - what exactly should I advocate when I say the government shouldn't enforce it?

Should I say that the government should spontaneously decide not to enforce this one law? Besides the obvious problem of it remaining on the books unchallenged, which would have made the process of overturning it even longer, it seems directly contrary to the rule of law to support laws going unenforced because of one person's view of morality. What happens when someone convinces the SEC it's immoral to go after ICOs because the growth of cryptocurrency is important to humanity, or whatever? What happens when someone convinces the IRS it's immoral to enforce tax laws against business owners, based on the morality described in Atlas Shrugged? Were I to support the principle of not enforcing laws that someone calls immoral, I suspect it would not actually achieve my political goals.

Should I say that the government should get out of the business of enforcing all laws about a certain subject? That also would go poorly - in the case of DOMA, I'd be saying that the government should not regulate marriage at all and should refrain from enforcing laws that ban child marriages, non-consensual marriages of adults, polygamy, immigration fraud, and so forth. I won't say that.

Should I say that the Constitution should be amended to enshrine my view of morality? Sure, but I think that's an extremely arduous process, and I think it's great that it requires so much of the country to agree on it, because, again, I don't want to be subject to other people's view of morality. I'll happily try to convince enough people that we should amend the Constitution, but it's easier - by design - to convince enough people that we should just repeal the law.