And don't get me started on girls wearing boxers. Yeah, I can see the kink appeal of a girl wearing her partner's underwear (even more so than their outerwear), but in my opinion, a girl just doesn't look appealing in men's underwear. But it's a popular trend that's almost unanimously deemed "sexy", thanks to mainstream "guy culture".
But, try turning the tables for a second. I think a guy wearing his girlfriend's underwear is sexy, too (from the perspective of a guy wearing girls' underwear, not necessarily how he looks). And it's not uncommon, because a lot of guys (though most of them probably wouldn't admit it publicly) do have a bit of a perverted interest in women's undergarments.
But how is it viewed in mainstream culture? Not sexy. Not even cute. Just plain perverted. What kind of guy wears his girlfriend's panties, unless he's a perv or a closet homo (or both)? Either that, or he's a "pussy-whipped" sissy and it's part of his humiliation at the hands of his dominant girlfriend.
I'll grant you, that men's underwear (at least of the baggy, boxer variety) is more comfortable than a lot of women's (tight-fitting) underwear. (Although, I've never liked the feeling of wearing loose undergarments beneath another layer of clothing). And while women's underwear is more frequently designed to look sexy, because of the difference in men's and women's bodies, it often doesn't have the same effect when stuffed with a bulging package.
These pictures are probably unfair, given my ultra-feminine style and grooming.
Nevertheless, as with the entire wardrobe, women's clothing is designed with more variety, frequently more emphasis on aesthetics over comfort (compared to men's clothing - except maybe in the case of tuxedos and business suits), and many more fabrics and colors and styles. Maybe most men aren't into those things, but just as some women probably love the comfort and simplicity of men's casual clothing (the kind of girls who take every chance to borrow their boyfriends' clothes, and often buy garments of a less feminine nature at the store), I know for a fact that there are some men out there who find the ethos and philosophy behind women's clothing appealing.
Now, you can find fitted tees for men, and skinny jeans, and things of that sort, that are probably closer to feminine styles than what guys usually wear. But even then, I don't think we get as much variety in terms of colors and fabrics and graphics and what have you. Girls have the option of rocking distressed jeans and Nirvana tees (and are respected for it), but guys aren't really given the choice to wear unicorn tees and sequined jeans (and if they do, they're more likely ridiculed for it). Unless they actively shop in the women's section, of course, but then finding clothes in the right size can be difficult - since girls can rock the baggy "one size too big" look probably a lot better than guys can rock the ultra-tight "a couple sizes too small" look.
I came upon that problem trying to squeeze into this dress.
Well, this isn't the first time I've conjectured on a clothing line for men designed to simulate women's clothing. Ultimately, though, what's needed is not just the opportunity for men to dress like women (if they so choose), but also acceptance for it as well (although acceptance may be a gradual side-effect of there being more opportunities). A girl can grab something off of her boyfriend's floor and wear it for the day without much to-do. But can you imagine what it would be like for a guy to grab something out of his girlfriend's closet and, in all seriousness, wear it for the day?