Yes, it’s true, and definitely annoying when you’re struggling to concentrate and embarrassing until you learn to guard your eyes. But it’s natural — meaning hard-wired, coded on our DNA.
Evidence is found in pupillary response, the way our eyes open wider when we see something that we like. (That’s why good poker players watch your eyes as you look at your cards.) Research in the mid-20th Century found that men’s pupils would dilate for images of nude women.
What images produced the same effect in women? Nude males? No. Babies.
It doesn’t mean the average man wants to undress every woman he sees, any more than it means every woman wants a baby. It just means biology is part of who we are.