Underwear.

Or pants, if you're British. (If you're American, educate yourself--in England, what we know as pants are referred to as trousers. Forget this at your own risk. You will be laughed at.)

For men:

You must never, EVER wear y-fronts.

First of all, they are perilously unsexy. I find them to be an instant turn-off, myself. Many other women feel the same about those so-called "tighty whities." Secondly...if you have plans along the lines of fatherhood, they're not exactly advocates for your cause.

Briefs are slightly better, especially if they're black (although not everyone will agree with me).

Boxer briefs are pretty much the tastiest things you can wear, if you want to show off your package but still look 'manly.' (Grey or black is lovely.)

I prefer straight boxers myself (so comfortable, so airy, such easy access), but pretty much anything other than the y-fronts are fine.

Thongs are RIGHT OUT.

And this is an interesting dissection of society's view of underwear.