Why? Two reasons:
It's a health issue. You realize when you sit on the bus, or any public seating area, there is only a thin layer (even thinner if you're wearing a thong - cause clearly panty lines and tights are like oil and water - or no underwear at all) between your private parts and those of that naked homeless dude that was sitting in that seat before you? Think about Dave Chappelle's real life experience on the bus next time your booty goes lululemon first:
The second reason:
It's a slippery slope. It's just like how skinny shorts - ahem - capri pants are turning men into a bunch of Leroys. Next thing you know, men will decide that lululemon yoga pants are pretty comfortable and they don't need to worry about wearing gloves to avoid zipper cuts when they're playing tap tap the ballsack.
Then, the slope gets even slipperier.
Ladies, do us men a favour and put on some pants. One morning you'll wake up and realize your man has worn your favourite high heel pumps to work to go with his matching skirt and lip gloss.