I think Chris Orr gets it:
"One of Juno's sharpest elements is its treatment of the Lorings. When we first meet them, we are obviously intended to like hip, ironic, artistic Mark and to find reliable, earnest, domestic Vanessa annoying and/or pitiable. What's impressive is the way the film gradually reverses our early affections, but does so without ever really changing either character. Instead it merely shifts our perspective, showing that the guy you want to swap mix tapes and spend afternoons watching horror movies with is probably not the guy you want to be a father for your child. In Knocked Up, the former abruptly, quasi-magically becomes the latter, allowing viewers to have their cake and eat it, too. In Juno (and, I think, real life), one not infrequently has to choose between the fun guy (or gal) and the responsible one, and it's a choice Juno does not hesitate to make."
In a tangential fashion I think this is exactly the reason why people have a hard time watching Bill Clinton on the stump for Hilary. He's still a 'cool' guy, but we realize that he turned out to be a sorta irresponsible prick, and, it looks like he's asking us for the car keys again through his wife. This puts Hilary supporters and liberals in general in an awkward spot.
PS: Just realized that if you haven't seen the movie this whole post probably doesn't make any sense to you. Well do yourself a favor and go see it. It's a lot of fun.