Thinking on the assumption that certain stories are true, and aliens in question are bipedal, almost identical to us I can think of three things.
1) It is anthromorphism, the way God is referred to as He or Him, or Jesus portrayed as a caucasian - just humans imposing their pre-conceptions on an idea.
2) Geoff's idea - that it is us from the future - perhaps after generations of genetic engineering. Would explain why they don't ever want to make contact, why they don't mind being seen on occasion (because they know from experience that the idea is laughable in the present) Hey maybe it could even explain drunken spaceships - a wriggle here n there seems much more unbelievable!
3) Perhaps evolution, of which we have only Earth as a guide, often follows similar paths - producing successfull designs.
Whats wrong with pondering? Good for a laugh - as long as you don't take it seriously. Personally, I wouldn't expect aliens to look anything like us (there we go - another assumption). Just because someone ponders these things doesn't mean they are loco - or that they will believe everything that they hear - they are only doing what humans enjoy doing. Wondering.
cheers