Obeded the 2nd wrote...
But women are like that in all mediums, not just games.
First off, I think that's a poor reason to keep doing it. But it's MUCH more pronounced in games because you can literally get women with unrealistic proportions and wearing outfits that defy gravity.
Here's a shot from one of my favourite (and quite mature!) games:

It also goes for men in movies, when was the last male protagainst a fat bald dude, never, who would watch that? same the other way around.
Are movies considered childish? maybe by a few but not by the majority which seems to be the case for games.
The problem
I have with it is it's typically male wish fulfillment. Having said that, it's much more likely that a male character in a movie be added specifically to draw sex appeal for women. This is very, very rare in gaming.
Having said that I think you are right, that may play a small role but I don't think it is a major part of why I'm looked down on for playing games.
It's just the stigma. Industry is still quite young. More and more gamers and growing up though.