For me... it's... I don't know for sure in fact. I never quite thought about why I mostly (not only) play female characters. And I'm not talking about BioWare games specifically, but ANY game that allows me to choose the gender (even if I don't get to customize the appearance).
What I DO know, however, is that as far as the Mass Effect trilogy is concerned I did create two Shepards (in ME1) the first time I played it. I played with both characters for about two or three hours each (reaching the Citadel, basically) and then determined if whether or not I liked the specific combination of my character's appearance with the voice actor's performance. Specifically for ME1 I did not "choose a gender" but rather "chose a combination of preferences". So I decided to go with a female Shepard because I loved Jennifer Hale's performance and it also happened to fit to perfection with the appearance I gave to my FemShep (I mean Hale is definitely more on the "dramatic" side in many lines she delivers, and my FemShep had that sort of expression on her face that would fit such scenes perfectly).
But that's just for ME1 specifically.
For ME2 I just merely imported my FemShep to continue with her (my 'canon' playthrough) and not because "I wanted to play as a female" (simply because my FemShep from ME1 was my 'canon' playthrough). In fact I created an alternate (non-canon) MaleShep in ME2 as well (he died at the end of that playthrough though... * sniff *). For ME2 I actually enjoyed my whole playthrough with that male Shepard I made (and much preferred Mark Meer's performance in ME2 especially as a Renegade Shep, which is the path I took with my BroShep). For ME3, again, I merely continued with my canon FemShep from ME1 and ME2 (and also because I thought that generally-speaking I like to see a female romance with Liara; sure I liked Traynor too but I just 'feel' in my gut somehow that if there's "a girl out there" for a FemShep it's Liara and no one else, oh well).
Now comes DAO and the reasons changed when compared to ME1. The thing is the Warden is NOT voiced in DAO, which for me was quite a big deal. So what I did instead for DAO was to go with a gender that would "fit a specific Origin AND a specific class". For some unknown reasons even to myself I merely "thought" (out of the big blue) that for example a female Warden would mysteriously "fit" better with a Rogue Dalish Elf Origin, while a male Warden would find himself at home coming from a Warrior Dwarven Noble Origin. I KNOW... it doesn't necessarily make sense (a male Warden could totally fit with a Rogue Dalish Elf Origin for some people but... strangely in my book it was just a big no no, and I don't quite know why myself; it just felt "natural" I guess). I had three Wardens total for Origins (and two for Awakening), two of which were male Wardens... but... my 'canon' Warden was * drum rolls ......................... * a female Rogue Dalish. I simply "clicked" and connected with her RIGHT AWAY, it was almost magical. I also "clicked" with my male Human Noble Warden but not "enough" to turn that one into my canon playthrough.
Then comes DA2, the first DA game with a voiced protagonist, so I was happy. But the way I did it with DA2 was slightly different in comparison with my method for ME1. I went with a combination of voice actor performance + appearance again BUT... that time around I also considered the whole "Diplomatic" / "Sarcastic" / "Harsh/Douchebag" personality differentiation. So I went with a sarcastic female Mage Hawke first, and then a 'harsh' male Warrior Hawke after. I ended up 'canonizing' BOTH playthroughs because I role-played them very differently and took significantly-different decisions as well (not to mention romancing Isabella for one, and Merril for the other).
And because I enjoyed both genders from DA2 I decided to create two canon World States for Inquisition. So for Inquisition I "decided to play male and female" simply because I ended up enjoying both playthroughs I had in DA2 with both Hawkes. But then again as far as my Inquisitors are concerned.... well, the method this time changed very little from DA2's. I considered the voice actors and actresses' performances, along with my Inquisitor's appearance and the "type" of Inquisitor I wanted to play (the personality variants). So because I have two World States to consider I went with a male and a female Inquisitor again, so no "gender preference" this time around.
Now, why did I say at the beginning that I mostly play females? Well that's because I skipped ALL other games I play outside of BioWare's library. In pretty much 9 times out of 10 in most MMORPGs I've played (or still play today) I mostly have female characters (be it "main" characters or even "mules"). For example, the only male character I have in Diablo 3 is the Barbarian because I just can't imagine a female Barb for the life of me (I know they're there, they exist; I see them occasionally but... it's just... nope, sorry, can't), while ALL my other toons are females (I SHOULD have gone with a male Monk though, because the female Monk's voice actress is just horrendous). In TERA I have 7 female toons, 1 only male (mule). In RIFT I made 5 female toons, no males. In Guild Wars 2 I made 4 females, 1 male. In Allods Online (which I do not play anymore; quit that one like 2 years ago) I had 4 female toons, no males. When I tried Wildstar recently (about two months ago) I made two female toons (one for each faction), no male. I also made 6 female toons (and just 1 male) for Champions Online. And it goes on like this forever... and I'm not exactly sure why. I mean I know why when it comes to BioWare games, but the rest... just can't put my finger on it.
Now, of course, I could limit myself saying "because b00bs". It's really more complicated than that in my mind. And, really, in some MMOs male characters can look absolutely handsome; to the point where one would go gay for them, like... really. So the whole "because they're girls and have fancy feminine curves" argument and reasoning to me is pretty moot. Sometimes I just don't even ask myself "Male or Female?", I just click on female and that's it (and I don't even ask myself why later... I just go on with it and start leveling up as if the male option wasn't even there at all).