Tell you to "steam off"? No such luck!
In the spirit of enquiry, I'll answer your question. The short answer is "Because I can." However, <Jedi>this isn't the answer you're looking for.</Jedi> So, you'll have to sit still for the loooong answer.
While I haven't been on the gaming scene as long as you have (I started in 1980 at the same age as you did), we've both got a lot of experience under our belts and have probably been around the block more than a few times - gaming wise at least! In my time I've played humans, elves, dwarves, mutants, superheroes, super villains (that was a wild MSH campaign!), wizards, warriors, rogues, barbarians, toons … If I haven't done the lot I'll be surprised. Twisted mutant weirdo? Check. Brain in a jar? Check. Starfish alien? Only if Hivers count.
I've also been a referee for a lot of games. As a result I've had to play NPCs of all stripes: both intelligent and non-intelligent. The NPCs, without a doubt, outnumber the PCs. After all, you can only play one PC at a time in a session. However, a referee has to deal with multiple NPCs. And, in all that time, some of my NPCs and some of my PCs have been female.
When I started out, I played in exclusively male groups. The players were male, the PCs were male and the NPCs were (with very few exceptions) male. Hell, I even went to an all male school (gender segregation is still big in the Northern Ireland education system). However, when I left my home for university, I was introduced to a wider world. I still kept up my gaming, but there were females! While the girls didn't outnumber the boys, there was a ratio of 2:1 boys:girls in the gaming groups at university. As a result, I found myself having to balance my refereeing more. I started including more female NPCs in my games, mainly as a response to the female PCs, and I got criticised if my female NPCs did not measure up to certain standards. I literally had to raise my game.
Post-university, I found the ratio of females to males in my gaming groups increasing, until there was one fateful group where I was one of two males in a group of seven! Ever heard of peer pressure? Well, when you're gaming with a majority female group (one of whom was my wife!), you get a lot of it. Ultimately, I ended up playing my first female PC (an AD&D enchantress, IIRC). This time I was really up against it. I was playing a female PC as a male in a group of female players.
Now, in most game systems it doesn't matter mechanically whether you are playing a male or female PC. Referring back to the wisdom of the Traveller LBBs: you can play any race or sex with the dice rolled. The scores rolled have no bearing. Where it matters is in the roleplaying. And that's the key to the matter: the roleplaying.
When I game, I game for recreation. I like to be somebody else, living in an imaginary world. I want to try on different personalities and see what fits. Today I want to be a half-orc barbarian. Tomorrow I want to be a tentacled mutant. The next day I'll be something else. To me, playing a female PC is just another set of clothes to try on. No more. No less. I am used to playing female characters, and it's not something I consider unusual.
Now, why did I decide on a female Vulcan engineer? Well, I'm an engineer for a start. I chose my career based on the encyclopaedias at my grandmother's house, and because I admired Lt Commander Montgomery Scott. It also helps that James Doohan's parents used to run a pharmacy in my home town (hint! hint!
). I chose a Vulcan because Vulcans are the alien race that makes Star Trek for me. Now, why a female? I chose to play a female because I wanted to add something to the character that was not stereotypical. Most people have an idea of how a Vulcan should be and will play accordingly. However, I'll lay odds that most of them will play a Vulcan male. Playing a Vulcan female gives me a handle on a character, makes it different and presents me with a chance to play something that isn't me.