I honestly wonder why people even care about underwear.
Like, no one sees it when you're outside, and once the action begins you take them off as soon as possible.
I'd much rather see him with a toolkit, some screwdrivers, not even for sexual reasons, just because that's actually useful.
Can't screw or drill with underwear, quite literally.
Well, I personally don't care if it's brand or not, but if it looks nice, that's cool. I do think it matters a bit, if a peek of it can be seen as a part of the ensemble. Like the color can matter. If your outfit is otherwise banging, but your completely wrong underwear is showing... But many think fashion is trivial, and that's a valid opinion too.
But I do think your underwear tells something about you.
Where it matters a bit more, is if it's raggedy. Like ok, if you really can't afford to buy new ones every once in a while, then it's understandable. But the cheapest ones are a few bucks so... To me it might tell something about the level of self-care and the lack there of, if it has ten holes and other wear and tear.
Or like if it has pictures of a cartoon character or jokes or something, that can say something about the wearer too. Or not, maybe they just put the cheapest thing on or their mommy bought them a pair.
Or it can say you have too much money, but not that much really, and you're trying to prove something. The really rich aren't rich by throwing money away on something just because it's a brand.
So no, maybe it doesn't matter in the grand scheme of things, but still.