Agree with Theii. Don't think "supporting" a constitutional amendments is a big deal since all the adults in the room know it's never going to happen.
This may be true, but there aren't many adults in the room, and by the "room," I mean the electorate in general, not just the GOP primary voters. Few know the real consequences of potential changes in the law. (For example, how many people think that overturning Roe v. Wade would outlaw abortion nationwide? Probably most, even though it actually wouldn't outlaw it anywhere but simply allow states to decide the issue as they had for almost 200 years.) We aren't a very deep people, and I don't see most of them examining his position with any nuance. They'd just see him as the guy who wants to amend the Constitution to ban gay marriage.
I favor gay marriage because I think it is a social good but I'd no more expect a Republican presidential candidate to be in favor as I would a global warming position consistent with the preponderance of scientific evidence. A candidate y gotta win in the primary and being reasonable on gay marriage, global warming or evolution would leave them dead before a race begins.
Like I said, the media isn't going to let the GOP off the hook on gay marriage, especially after the Party spent the 2000s exploiting the issue. They love making Republicans have to answer for positions the most hardcore elements of the base take that are at odds with most Americans. (They could do it to Democrats, but they're not going to, because they're Democrats themselves, and they know it would harm their candidates.) However, the candidate doesn't have to help them.
Walker could have said he personally believes in traditional marriage but that the issue is going to be decided by the Court and left it at that. If the Court rules in favor of gay marriage, it could tremendously help the GOP by letting them get out of the issue gracefully, but it is possible for them to screw that up, and Walker is doing it. He didn't have to bring up the constitutional amendment issue, which forces the GOP candidates to dig in on the issue and let it get polarized. And keep in mind that the President doesn't even play a role in the constitutional amendment process. Amendments get proposed by Congress (and aren't subject to a presidential veto) or by constitutional conventions called by the state legislatures. Neither process involves the President, so there is no reason why either should have to take much of a position on the issue.