Well, it's not always about what's practical; it's about what's most comfortable or what a particular person enjoys wearing. Some girls pull off a dress quite nicely, while others actually look quite bad in them. But no one thinks of a girl as doing something unnatural if they wear a dress (no matter how unflattering it is on the girl in question). The same isn't true of men. People don't just say, "Wow, that guy doesn't look good in a dress." They say, "Can you believe that guy is wearing a dress?"
People who are otherwise perfectly nice individuals become a stereotypical high school "popular girl", scoffing at the very idea of a man wearing a "girl's" outfit. It's crazy how fast the shift is. My mom actually did this very thing last week when I was getting my hair cut. We were in a styling saloon, waiting for the stylist who was going to take me to finish with her last client, and apparently a man was walking around, browsing some of the hair supplies in another part of the store, and she leaned over to me and whispered, "Did you realize there's a man walking around here wearing women's clothes? Now that's just scary." And I had to just shrug it off because I honestly hadn't noticed, and obviously don't particularly care. I don't think I've ever heard my mom talk that way about anyone before, and it really took me off guard because she's always so nice to everyone.
Another example: a friend of mine from college. Last semester, he attended a special event based around the movie Rocky Horror Picture Show, and he decided he wanted to cross-dress for it. So he got dressed up in full drag. Black dress, small heels, black stockings or leggings, the whole nine-yards. And quite honestly, he wears a dress far better than probably half the girls I know (and that's not at all an insult to the girls I know). He already wears the whole "goth" type attire on a pretty regular basis, so he's pretty familiar with eyeliner, mascara, nail polish, and the like, but he is in fact perfectly straight. Yet, there are plenty of people (like my mom) who would look down on him as a person (not that he cares, but still) simply because he likes to wear a dress every now and again. And that's a problem.
"When you're falling in a forest and there's nobody around,
do you ever really crash or even make a sound?"