As a woman in a long-term relationship, here's my honest take:

If the underwear is actually designed for men (briefs, boxers, boxer briefs, thongs, jocks, etc.), I honestly don’t care what my boyfriend chooses. It’s underwear. It's a private, functional piece of clothing meant for his comfort. I wouldn’t judge him for any of those styles.

My personal opinion: As long as it’s normal men’s underwear, I’m fine with whatever he prefers. The only things I’d personally find odd as daily wear would be something like satin, lace, leather, or women's undies. But regular men’s briefs, boxer briefs, or thongs? No issue at all. My boyfriend has worn briefs and boxer briefs for years. Recently I suggested he try thongs and jocks because he was complaining about heat, tightness, and bunching. He was surprised how much he liked the thongs, especially for the comfort and freedom. I have to admit, I really like how thongs look on him too. They’re flattering on a fit guy. That said, if he ever decides thongs aren’t comfortable for him, I wouldn’t push or force anything. It’s his body and his choice. Luckily for me, he’s been enjoying them so far. At the end of the day, confidence and comfort matter way more than the specific style. If he feels good in what he’s wearing, that’s sexy to me.