Well I don't respect the argument. I don't think theirs is a defensible position and I think it's unfortunate that they believe that way, and I'd prefer it if they'd change their mind. But I'm not going to start disrespecting them as a person for that one view.
However, one belief like that, coupled with many others that are unreasonable? At a certain point it becomes hard to no longer judge a person along with their viewpoints, if I find their reasons for believing them are very weak, and they refuse to acknowledge weaknesses in their views. Especially when their beliefs are ones that are aggressive towards the desires of other people in ways that are not zero sum.
One of my oldest friends is a rather conservative person, and while we don't see eye to eye on everything, he's often willing to admit or see the logic in my viewpoints, and I treat him with the same respect. Though, what typically happens is, he recognizes the strength of my argument, then says he hasn't done research/doesn't know enough and kind of backs down. I think he doesn't want to really question his beliefs down to the core, which is fine, that's how humans are, challenging core beliefs is very difficult. I don't press the matter out of politeness, but I'm pretty sure there isn't a foundation under that house, and if he would ever thought about it long enough, he probably would be confronted with some contradictions.
A good example: socialism. I can appreciate a person who argues socialist governments have been some of the worst, and holds a viewpoint that even idealistic socialism tends to lead to bad things, and recognizes Pol Pot, Lenin and Mao were terrible. I think anyone who doesn't recognize how awful the Soviet system was can't be serious, the terror was pretty objective. But, that sort of view also gets taken to a pretty stupid rhetorical standpoint where all reason and history gets thrown out of the window. People forget to compare the scale of societies, like how many people died under Mao's rule, but forget that China even then had a population of ~600 million and had just underwent civil war. If someone tries to peg all of the problem on just socialist ideas, but refuses to recognize the role that historical circumstance played in 20th century socialism, then I don't think I can respect them. It's just too stupid to say "millions died under socialism" with no context, comparative study, or any attempt at a fair evaluation of the facts.
Going back to marriage, it also depends on exactly what they think that belief on marriage should entail. Someone can prefer if gay marriage wasn't a thing, but there's a difference between having a preference and advocating for policy to enforce that preference. A person who believed in gay marriage with reasoning I think is poor, who also puts in money, time or effort into working against gay marriage legally, I would have less respect towards, because beliefs made politically active are more consequential than those which aren't.
A person who opposes gay marriage and also doesn't believe a bunch of other intolerably awful **** is kind of a unicorn though, so I will say the odds are against my having respect for a person who believes that.