Yoga pants are indeed a wondrous garment, as far as extremely informal wear goes! They're an almost perfect confluence of form and function: as other commentators have noted they're actually pretty comfortable attire for women to wear and workout in, and the fact they hug and show off such amble curves as the above photos demonstrate is really a great bonus for the male gaze!
And while men also deserve to have similarly comfortable work out wear that also has such sex appeal, I would agree that hugging their bodies in the same way that yoga pants hug women's bodies is far less appealing proposal! I know that I wouldn't have the self-confidence to wear such a thing!