There's no I in Team Fortress
First person shooters are not exactly known for their feminism. Hell, they're not really know for much of anything apart from being the domain of immature teenage boys or a scapegoat for the more hysterical parts of the right-wing media. Team Fortress 2, Valve's free to play juggernaut, avoids at least one part of that with a large helping of crazy cartoon violence, and doesn't get mentioned as often as Call of Duty, for example. Unfortunately, the immature teenage boys remain.
You run around and shoot other players with your gun. So far so ordinary, you think? You'd be wrong. TF2 is legendary for a reason. There are plenty of class-based shooters around, but TF2 is the one everyone knows.
Nine classes, hundreds of guns, and thousands upon thousands of possible playstyle configurations, and the game is so finely balanced that no one class or configuration is an absolute killer. Usually it's a case of 'strong against this, weak against that', and it's impossible to be completely hardened against every other class. Sort of like rock, paper scissors.
So, what to play as? You have so many choices, all equally good, that you can choose your own playstyle. Sneaky-backstabby-squishy is the Spy. Slow-lotsa-damage-tough is the Heavy Weapons Guy. Like one-shotting people form half-way across the map? Play a Sniper. Each of the classes has a personality and a lot of awesome one-liners that they say at random, and some classes work very well together.
That's what it's all about, really. If the game pushes you into any particular playstyle, it's this: you MUST work together with the other players as a team, or you will lose.
Cartoony, silly as all hell, and it's actually aged pretty well for a game that's six years old at this point. One really interesting thing to note is that the game was in development for nine years because Valve spent so long on the design of it, and they certainly knocked it out of the park there. TF2 is also part of the Steam Workshop, where players can design their own items and submit them to Valve for possible inclusion in the game, and I don't know of any other FPS that even considers doing that.
Can you play a female character?
No. All of the nine classes are male.
TF2 has a very active modding community. An enterprising individual by the name of Chemical Alia created a number of female skins for TF2, and a few other enterprising individuals did the voice acting for them. With some trickery, you can play as a female and see other female characters in the game.
Are the women sexualized?
There are very few actual female characters in the game's canon. It's worth noting, however, that the one in control of just about everything is a woman with super-villain levels of awesome who looks like this:
Valve doesn't bring the sexy the way other games companies do, and they deserve all the cookies for that.
Is there a similar level of customization for the female characters as for male characters?
Are the numbers of NPCs balanced between males and females, and do they have a similar overall level of power?
TF2 has no NPCs. It's always player against player. Of female characters in general who are a part of the TF2 universe and revealed through comics etc, there's a bias towards male characters with a similar bias in power.
None. This is an FPS, after all, not a roleplaying game. That said... Valve are sneaky as shit.
So - TF2 has a huge fanfiction community. This community is all about the relationships, mostly between the male characters, and mostly written by women (as much fanfiction tends to be). And hoo boy, do they ever turn up the knobs on the whole homoeroticism of the game.
Valve are aware of this, because good grief, how can they not be? And they like to tease. The game has any number of weird hints about gay relationships, including stuff like the Beaux and Arrows achievement, and some of the character lines. The hints can always be read both ways, so the players can never really know.
If you're going to experience sexism in TF2, it'll probably be a result of the players you're fighting with or against, not the game itself. Apart from a low-grade bias towards male characters, the game is pretty benign in that respect as well as being hella fun. I give it a thumbs up, with the caveat that you need to carefully vet the servers you choose to play on, and make sure that your fellow teammates won't react badly to hearing a female voice over the game chat.