I agree with Awful Falafel. "Girlfriend Mode" aside, I found Borderlands to have surprisingly strong female characters all around. You could say that Moxxi's "predatory sexuality" plays to a certain kind of trope, but I think it speaks to her pragmatism more than anything: she'll use any resource she has to get what she wants. In men, that's often called "ambition."
But keep in mind, Borderlands is a game that's about shooting stuff and you never, ever are asked to shoot any woman. Ever.