Thoughts on the reasons stated:
1) Attraction: Oh my god, it's a girl and she has a brain
and is interested in some of the same things I am
interested in. Maybe I can get a date!
I wouldn't put it past the majority of my gender.
2) Misguided attempts to encourage women to post more: Oh
my god, it's a girl and we need more girls posting here.
Let's be nice and friendly and make her feel all welcome
and stuff so she will post more (thereby sucking all the
intellectual content out of their posts and killing any
reason you have to post there).
I wouldn't put it past HN.
3) It's not really kid gloves, it's really walking on egg
shells: Oh my god, it's a girl and what if she breaks
down and cries because I argued with her (like my
girlfriend routinely does)
Now
that I just can not see.
3b) or what if I make a joke and she takes it all wrong
and everyone will think I am some horrible sexist pig!
People have lost their jobs to that one. Even I'm doing that. You would be crazy
not to do that
4) Shyness: A lot of the very intelligent men you find
hanging out in certain types of online forums (like
Hacker News) are very introverted and just awkward around
women and some of them have gotten a lot of rejection, so
they get nervous once they find out it's a girl.
iffy.