I'm mostly a lurker here, but I've been following this thread because it's close to my heart, so I thought I'd share my story...
I've been on SL for over 5 years. When I first came here, I created a male avatar. I started checking things out, but quickly got bored. About a month later, I created a female avatar, and it was a completely different experience. I loved my female alter-ego, and had so much fun trying different skins and hair, dressing her up, etc. Okay, I know that doesn't sound unusual, so why am I bothering writing this? Here's where it gets interesting...
Well, I guess I should have made it clear from the start that I'm AMAB (assigned male at birth... biologically male). Around the time I first came to Second Life, I was at the tail end of a failing marriage. Long story short, I'm divorced 3.5 years now and after a lot of introspection and soul searching have discovered I'm genderqueer / genderfluid / trans-feminine. If you don't know what any of that means, essentially it's that although I'm not a transgender woman, my gender identity falls somewhere in-between female and male. Normally I'm closer to female than male, but it actually isn't a static thing (hence "genderfluid").
I'm still learning about who I am, but now that I've started to investigate this stuff, I've realized that I've been this way since I was an adolescent. Back then I shrugged it off as youthful experimentation. As I got older, I stifled that part of myself and lived a "normal" heterosexual male life... Met a girl, got married, had kids, yadda yadda yadda. Now that I'm older and more accepting of myself, I'm embracing who I truly am and have a lot of regrets that I wasn't true to myself when I was younger. I mean, I am in a way, but I also wouldn't change a thing... I have two wonderful kids, who I love more than anything in the world. Who knows how my life would have been different if I hadn't done things the way I did.
My kids do complicate my new found gender identity though. Since understanding, and becoming more accepting of who I am, I've been experimenting with a more feminine gender expression. Actually my profile picture is really me! Luckily, I work from home, so when my kids are at school, I can play "dress-up" and express my feminine side. I'd love to be able to be able to dress more feminine all the time, but my kids are adolescent aged, and that's a hard enough time as it is... the last thing they need is to have to deal with their dad dressing like their mom.
Anyway, that's my story, and that's why I choose to play a female avatar. I actually think that when I first started playing a female in Second Life many years ago, it helped me by getting me to think about why I enjoyed it so much... It was one of many things that led me to my introspection, and eventually to my self-realization.