It doesn't really bother me one way or another. I've had to watch men wear women's undergarments on cam (I work at a cam site) and it doesn't gross me out nor turn me on. If my boyfriend did it, I don't know. I might think differently since I'm actually attracted to him. I might like it. Well, I'd like him in anything so there you go.
And really, WE are the ones who decide what clothes are really "men's" and "women's" clothes so to say that a man wearing "women's" clothes is weird doesn't make sense to me. In Scotland, they wear kilts and it's considered a manly thing to do but here, men in skirts (and even kilts - I've seen them) are laughed at. So it's subjective, really, and people should just let people be however they want to be and not shoot down the opportunity to wear clothing designated for one sex or the other (like there is only two, anyway).
Finally, it is a bit offensive, when you think about it, how men are embarrassed to wear "women's" clothing. Women wear pants all the time. But those used to be solely for men. Men, at least in America and places like it, don't wear skirts. Why? Because it's "understandable" for a woman to want to dress like a man but "embarrassing" for a man to want to dress like a woman. That's a damn shame.
But I digress. This rant adds up to this, really: You go, Miss Nessa! An attitude of indifference (or even admiration) is the perfect response to have towards a man wearing "women's" clothing (underwear and otherwise) because there's nothing wrong with it!