A guy I don't know well sent me a creepy Facebook message with pervy language. Next, he messaged me a bunch of tantra memes -- sex as a celebration, blah, blah, blah. It grossed me out. Why would a guy think he can be so blatantly sexual out of nowhere? What should you say to a guy who does this?

--Yuck

When a guy messaging you starts sounding like Rumi or some other ancient elder, it's usually for good reason -- like that he's short on hookup partners and the market's way behind in building realistic washable sex robots.

It would be instructive for men who do this to consider sex differences in the appeal of unsolicited genital selfies -- sent, for example, by strangers on dating sites. The Kinsey Institute's Justin Garcia reports that only 5% of women are aroused by unsolicted penis selfies; the vast majority are just grossed out by them.

As for the reception vagina selfies get, a Los Angeles woman sent 37 men on a dating site an unsolicited vagina pic (not hers, one she found on the internet). Three men replied with shirtless pix; seven sent messages about what they'd like to do to the pictured vagina; eight asked for more pix; nine sent penis selfies; and one sent a video that the woman told Metro UK included "a, um, happy ending."

The difference in men's and women's responses to "down there" selfies from strangers makes sense in light of how female emotions seem to have evolved to protect women from becoming single mothers -- getting knocked up and then ditched. Research by anthropologist John Marshall Townsend suggests that female emotions push women to look for signs of commitment from a man, even when they know they want nothing more than casual sex with him. This, in turn, probably leads many or most women to be put off by overt sex talk from a man -- before there seems to be an emotional connection.

Yet, perhaps due to what anthropologist Donald Symons calls the human tendency "to imagine that other minds are much like our own," many men whip out the sex talk and the zipperwurst pix for women they barely know. If a guy who does this is some Tinder rando, you can just block him. But when it's a male friend or other guy you'd rather not cut off entirely, you need to be straight with him -- like, "Dude, from now on, you gotta keep any messages totally platonic" -- and be straight with him again if he tries again. (I mean, come on...if you wanted gross unsolicited sexual comments, you'd wear a halter top and booty shorts to 7-Eleven.)

For pages and pages of "science-help" from me, buy my latest book, "Unf*ckology: A Field Guide to Living with Guts and Confidence." It lays out the PROCESS of transforming to live w/confidence.