It's none of someone's business why unless you choose to volunteer that information, and needing to know why you've just been turned down is a massive low-self-perceived-status signal.

Okay, seriously? This kind of "No you can't know what you did wrong, asking means you're even lower-status" dynamic to sexuality has probably been responsible for a number of geek/Aspie suicides over the last century. The existence and popularity of PUA isn't so much a response to men who feel deprived of sex, it's targeted at men who feel deprived of sex and romance and any idea of what they're doing wrong and any known strategy for even getting started on fixing things. A major reason why people hurt is that there's no known gentle slope into sex, and not getting any feedback is part of that.

I've informed a number of male college students that they have large, clearly detectable body odors. In every single case so far, they say nobody has ever told them that before. (And my girlfriend has confirmed a number of these, so it's not just a unique nose.)

If you don't need to ask yourself, that's fine. If someone else does need to ask, try to be more sympathetic. And if someone asks you, TELL THEM.