The female geophysical structure involves wider hips. The fabric of the garments in question is generally somewhat stretchy, which simultaneously makes it supportive and capable of making room for whatever it has to make room for. There is a mild tendency for some of the fabric that might sit against female physical architecture farther in the back to pull forward between the legs to a position more frontal. Mostly though these (Hanes for Her) don’t feel very different from these (Body Glove male bikini cut) when you have them on.
Here are ten reasons for why an otherwise normal guy might be in favour of wearing women’s undies. NOTE: I am admitting NOTHING. 
[ol]
[li]They are pretty! Most men’s underwear is downright ugly.[/li][li]Gift Wrap! When you give your lady a present, do you use paper decorated with power-tools or racing cars? No, you use a nicely decorated satin paper with lace trim![/li][li]Comfortable.[/li][li]“Doing things you can’t do, just because you can.” (An homage to Katawa Shoujo)[/li][li]The most efficient solution to a certain “Engineering Problem”.*[/li][li]Some religious adherents have their own special underwear. Now you can, too![/li][li]Lesbian fantasies?[/li][li]A way to get in touch with one’s “feminine side”.[/li][li]Giving the middle finger to the whole culture of Toxic Masculinity.[/li][li]That little inner smirk of looking at one’s coworkers and thinking, “I bet you don’t know what I am wearing right now!”[/li][/ol]
- The “Engineering Problem” I speak of is the one that is constantly on every engineer’s mind, especially while attending university:
Q: What is the quickest way to get into a woman’s panties?
A: Wear the damn things yourself!
I have an opposite anecdote. Years ago, I went to a costume party, and was shocked when a very shapely blonde turned out to be my friend Rick. He had gotten a slinky dress for the occasion, but being a tall and very lanky guy, he had no hips or butt to fill it out. So he put on like a dozen pairs of his own underwear to pad himself out. 
I know the OP is really old, but it would have struck me as weird even then. Isn’t part of the fun of wearing a costume getting the chance to feel like something or somebody you’re not? Why wouldn’t the boyfriend prance a little? If I, a woman, dressed as a lumberjack, I would stomp around for sure.
not much different for me … 'xcept she was more than just a girlfriend. and, i don’t consider it to be ‘transvestism’ or whatever freud might call it. s’been 40+ years since … no regrets on my part.
Fear of homosexuality, or maybe (these two seem inextricable) fear of women.
I remember a friend telling me about her guy having to wait in the women’s section of a department store for her to buy something and when they were leaving he hissed to her, “I felt like I was surrounded by fag clothes!”
At the almost unbearably popular adult Halloween street festivities in Santa Cruz CA, I would say at least half of the men in costume are in drag. It seems to me that the whole idea of women’s clothing has always been entirely about men. Women are otherized and fetishized by their clothing. So it makes sense that a costume men would be attracted to wearing would be a dangerous little step into the Other.
And although there is average, or common, there is no normal in sexuality, as far as I can tell.
Wow, zombie resurrected by a poster with a 2014 join date and 25 posts, also replied to by a user with a 2012 join date/28 posts and 2012 join date/290 posts in the first 20 replies? Interesting sometimes what prompts infrequent posters to join in! 
I think SailorHG and some of the previous posters are on to it. Women’s clothing can be more pretty than practical, and men’s clothing is frequently strictly practical. And men can enjoy pretty things too. It sounds to me like the guy in OP really enjoyed what he was wearing, and for all we know it wasn’t just for that party.
Though like so many things of this nature, there are probably many reasons a ‘normal’ guy would wear women’s undies.
With a few more days to ponder, this: ** For what reason do you even wear underwear at all? ** Before going on, be sure you have answered that question, thoughtfully. Now go on to the next question.
By what criteria do judge various models/styles available, to meet the needs above?