Not to mention that a large percentage of females find muscle mass attractive when combined with a low body fat percentage in males. Indirectly this can lead to a boost in self-confidence levels in an individual, which has other positive spinoffs.
This. Times ten. I can't stress this enough to my non-lifting, skinny, geek friends who wish they were more attractive to women. I always tell them that the number one thing they can do is to begin lifting weights. They'll look healthier, more athletic, manlier, and their self-confidence will go up. Oh, actually weights are number two. Number one is to ditch the anti-girlfriend geek t-shirts with programmer's inside jokes (you know, the 'only 10 people who know binary' stuff) ;-)