They only used one person because they simply wanted to know what parkour looked like in detail.. They wanted to know what a real wall hop looks like, what a real mantle looks like, so they used slow motion cameras on one of the best traceurs in the world. Women move differently because they have slightly different skeletons, most notably the hips. This is mostly seen in the way their legs move, but because you run and jump with your legs, women's upper bodies move differently as well. Many games don't bother giving females different animations, but the female characters very obviously walk and run like men, and it looks strange. It would be even more obvious with parkour, but only to people who know what a male and female look like while doing parkour.
A female's skeleton, especialy that of a girl, does not greatly affect the way in which they move (unless they're pregnant, but then why the f*** would you be doing parkour?). I have been doing long-distance running for four years now, and still many of my friends who I run with are female. Often when I'm conscentrating while running my vision blurs into the distance and, on more than one occassion, a have mistaken males that I do not know (I can recognize the movement of people that i know) for females and vice-versa (fact: the greater the distance in running, the less gender matters. Some ultra-marathons [i.e. 200-miles] have equal average performance by women as by men).
Anyway, there is no where near as much talk about this as i expected (having been through a crazy amount of forums on other games in the past), which probably shows that they (the people who usualy go ballistic about this sort of stuff) have simply discounted this game as a contender for their money as the majority of highly customizable triple-A games now have gender selection (i.e. Fallout, the ENTIRE Elder Scrolls series (except for Redguard which has a set protagonist), Diablo 3, Torchlight 2, etc.). Honestly, I generaly check for both split screen and gender selection (if it has customisable characters) before I purchase a co-op-featuring game as I often play with people who are girls in such cases. Also, as usual, it seems that people don't take the SINGLE PLAYER option into account, where they will presumably not be harrassed by the obligatory pile of jerks that comes with all multiplayer experiences that i have seen to date.