unfortunately i think the issue is more complicated and tied to so many things, but ultimately the idea and image of masculinity, toxic masculinity and homophobia.
too many straight men thing anything not a boxer is 'panties', and straight male dominated cultures [which is most if not all] tend to think of anything else as "less".
We and culture has to actively work on breaking down those barriers, like intimacy with friends [even I dont hug friends, thats something I have to work through], accepting bodies and nudity as normal and not shameful or sexy, and not judging others. We can do only so much with visibility and our own 'activisim' but it's going to take a culture shift, which is slow. and unfortunately, being more accepting of other people's underwear choices and expanding one's own is super low priority, in the state of the world right now
one thing might be to actively ask retailers corporate offices to offer more variety in their stores, offer accessible brands with large ranges maybe, to encourage them to give folks better access and instant gratification compared to online shopping. I know many retailers are losing the battle to Amazon and online shopping, and may be open to suggestions?
I agree a lot with you, it's true that gender stereotypes and homophobia are part of this problem. If you use are heterosexual and use thongs you're going to be considered gay -even worse, they consider to be gay as something bad.
Obviously, the end of homophobia is much more important than the end of prejudices of thongs for men, but I think there is more, there is less homophobia than years ago and prejudices in thongs are the same. I think many times gay people break gender stereotypes and heterosexual people after breaking those stereotypes begin to do what gays began before to do but I've got the impression that thongs are seen by gay people as heterosexual people do. I asked something relative to that when I started in this forum (
http://mensthongforum.com/index.php?topic=485.0) and most of answers were "similar to heterosexual men". The idea of that thread was due to a conversation with a gay friend who I wanted to tell (because I thought he had got less prejudices) I use thongs sometimes and when the conversation was about that, he said that's freak so I didn't tell him.
PS: In your country men don't hugh or dont wear briefs? That's very curious for me.