You sound like me...I'm just atheist, and in addition I don't come from any kind of religious background. I feel that a lot of these "religion replacement" traditions (like Sunday school) are because people grew up with religion and are not sure how to replace the social void when they're no longer part of that community. DH's family is pretty atheist, but they go to a UU fellowship and I don't think there's any "fruitiness" involved...they have speakers on various subjects and fun get togethers. Although, I remember when ds was born they wanted to do some naming ceremony with him, and I thought that it was giving it "spiritual meaning" that I personally didn't feel I needed, so we didn't do it.
It's definitely to fill the social void. Funny thing is, i never really felt "connected" when i attended my (christian) church. It's just being *part* of a group, i suppose. And i'm just plain lonely. Hell, i'd be happy just having a couple families come over for a potluck or bbq at a park once a month or so! I just want to make connections - just not deal with the quasi-religious liberal BS in the process though