ILuvChubbyChix:
No it's not just you. There really is something seemingly juvenile about roleplay and make believe. Adults are expected to understand the cold, hard reality and if they'd like to do something, to actually make it happen. There's daydreaming and being lost in thought, but that's different.
Hell, it really reminds me of when folks used to refer to "cybering" online. Mostly amongst those who are officially too young for this site (and I'm duty bound to ban if I discover any). I think the kids call it "sexting" now.
That said, I suppose you might find some folks of all ages who might do this if you search long and hard enough. While not too common, there's some adults who still have a sense of being a child at heart. I never had the desire to engage in online roleplay, and I completely outgrew any notions of make believe after I turned 12 or so. Even roleplay questions at some job interviews irritate me and make me think less of the company. But, some folks never really do outgrow this stuff at all.

If you aren't into RP, more power to you. No shame in not enjoying it.
HoweverRole-playing is as juvenile or adult as you make it. It can be a lot of fun if you do it well. People do it in a committed relationship as well.
Also, the name of the site is
Fantasy Feeder. If we are hostile to the mere idea of participating in fantasy, what is even the point of this site?