Some people get really weird about playing characters of a different gender than their own. I don't mean in an "I wish Assassin's Creed had playable female characters" way, I'm talking about people, mostly guys, who seem to think that choosing to play a female character makes a man less of a man. Look around on any MMO forum and you'll likely find the 'why do guys play girl characters, what's wrong with them?' thread.
You see a lot of reasons suggested, ranging from 'their gay lol' to 'they want people to give them stuff' and so forth. It's depressing, honestly. So many of the posts always seem to boil down to 'I don't want to do thing X so anyone who does must be wrong/different/weird'. There's a self-centeredness and lack of empathy that seems emblematic of so much of what's wrong with online culture.
When I create characters in online games, tabletop, or whatever, I generally have a basic outline in my mind when I start. I'll either have a class or a personality in mind and everything else flows from that. One of those elements is gender. I'd feel as strange trying to make a character male that I've pictured as female as I would the reverse. Playing a female character is comparable to playing a dwarf, or a robot, or a psychotic little cat thing for me. I'm not any of those things in real life, they're elements of a character I choose to inhabit.
That's not to say my views on characters are the way, the truth, and the light. I can absolutely understand wanting to play 'yourself' (or more likely an idealized version thereof). My version of the Avatar in Ultima games has always been simply me at the core. It's also not at all surprising that a transgender person would want to play a character of the gender they identify as. For me such a role is a challenge to attempt; for them it's an opportunity to inhabit a more comfortable skin.
I think a lot of my view on this comes from my roots in late 80s and early 90s tabletop role-playing. This was a time when story and setting was really coming to the fore, and games tended to involve a lot of social elements along with the combat. Like most players I started out pretty much playing 'me with magic', but that gets boring after a while. For role playing to be interesting, the role needs to require some effort. As one gets better at it, the effort needs to increase. Particularly once I started running games, I needed to be able to play all sorts of different NPCs, some of whom were female. From there to a female PC isn't much of a stretch.
Ultimately, choice of character gender is just one of many, many ways that different people play the same game in a different way. Trying to claim that someone else is playing wrong because of that choice says a lot more about you than it does them.