Decided to come out of lurkdom and reply to this thread as it is one of my favorite topics about SL and the reason I joined. Years ago I found out my RL husband was using a female avatar on SL so I wanted to learn more about why people did this. I am a female in RL but did joke about joining so I could practice being more feminine.
For the first year or so we hung out together, both as females, and people would assume that I was a male in RL and he was the female - we told people we met that we were married in RL. Well, we told people we were married in RL after some drama with a friend finding out when we hadn't told her at first. As I got more comfortable in SL I went out on my own more and met various people and enjoyed chatting with the girls who mentioned in their profile they were guys in RL.
I met a guy who played mostly as a female in SL and he admitted to wanting to meet a girl to play a guy so they could RP together. So, I decided to try gender swapping as well. To my confusion no one ever doubted my RL sex when I was presenting as a guy in SL, but I got doubted all the time when I use a female avatar. I wonder if it was because guys with female avatars are more common than girls with male avatars. Or do I naturally act more like a guy?!
One of the reasons I suspected guys might like to use female avatars is because it is more acceptable in RL for females to dress and act like guys, but still not very well accepted for males to act and dress like girls. In SL they can get in touch with and act out that side of themselves.