I'm honestly surprised that so many people are upset about Duke slapping a babe in the ass to reassure her that she'll be safe from the other team shooting at her? Or that people actually thought that Duke, the guy whole risks his life all the time to save babes, would actually slap them in the face?
I have a feeling that anyone who gets upset about this knows nothing at all about the Duke or the Duke-averse. In Duke's world, a slap to the ass from duke is pretty much the most assuring thing she could get while being hunted by aliens. In the Duke-averse, Duke loves the babes, and the babes love Duke, it's as simple as that.
I know, it doesn't make it less offensive, but the Overly Long Gag [http://tvtropes.org/pmwiki/pmwiki.php/Main/OverlyLongGag] is the essence of what Duke is. Taking him as anything more than a running joke means you're doing it wrong, very, very wrong!
The news will still rip him a new one, and it may even set video games back a bit, but seriously? He's a joke, and always has been; that's the entire point. Being angered by him is about as bad as being angered by a troll, or taking Chuck Norris jokes in the literal.
Play the game if you want to have that old juvenile enjoyment you get out of "that's what she said" style jokes, but never take those, or the Duke, seriously; you're only making a fool of yourself doing so.