Re: Heed the Warnings of The Ride...
You seem pretty resistant to all the suggestions as to why this could be offensive and probably got rejected, but the fact is, just because you didn't intend the design to be offensive, doesn't mean it wasn't actually offensive in execution. Don't use amateur artistic skills as the scapegoat, because it's not about your skills here. It's about the concept.
I agree with the numerous posters, that without the context of amusement parks in some form or another, this more or less appears as a sexual shirt or a discriminatory shirt.
I really like simple shirt designs though, personally, so I would encourage you to keep submitting in future derbies! Shirt.woot's rules aren't really written down anywhere, so there is of course alot of interpretation by the staff, but you'll get the hang of things. Here's the "unofficial derby rules" thread I found most helpful http://shirt.woot.com/Forums/ViewPost.aspx?PostID=1946963