First, I think you're right that a combo of youth and social inexperience (and general youthful over-exuberance) is to blame for most blunt invites. (Better than a true "blind" invite, where the invite pop-up simply appears without comment - joy.) They mean well, they just don't know any better.
I run a female elf when I run a TP farmer - it's a change of pace, something different than my mostly male alts (and better scenery than watching the backside of a dwarf). And since she only lasts a couple days I never bother to join my Kin with her, so she (in her various incarnations - close to 30 now?) gets "propositioned" to join kins on a regular basis. But my male characters are all in Kins, so... who knows?
And, yeah, you are a rather small sample pool, even "those people who respond here" are hardly a meaningfully valid cross-section, but I'd think that with some (much) younger and over-enthusiastic (and under-experienced) players there may be some gender bias. I don't think I'd be going out on a limb to say that it's a mostly male player-base, and (newsflash) males tend to be attracted to female examples of the species. It's quite possible that a "female form" grabs a male's attention just a little more, even at the unconscious level, and so those kin-less characters are propositioned more often without any intent behind it. <shrugs> But so what? I mean... whadya expect, some fantasy world where everyone's treated as an equal?
If it becomes irritating enough, there are many "zero-expectation" kins one can join, turn off /kinchat in your /chat box, and never look back. Good gaming!