That's according to the latest issue of OXM, which describes Duke Nukem Forever's four multiplayer modes that feel ripped from the '90s. They're variations on deathmatch, team deathmatch, capture the flag and king of the hill modes, only with Duke-ified names - "Team Dukematch," for example - and that aforementioned babe smacking.
The issue calls Capture the Babe mode "pretty much what you'd expect from a CTF-style mode with one small touch: the Babe will sometimes freak out while you're carrying her [...]at which point you have to hit a button to gently give her a reassuring slap," but assures readers "it's more goofy than offensive."
According to the PR firm for developer Gearbox Software, that "slap" is done directly to the keister of one's captured babe, presumably slung over the shoulder of your multiplayer Duke avatar.
The rest of DNF's multiplayer, supporting up to 8 players at once, seems full of familiar Dukeisms, like the ability to grow tall or shrink, wear jetpacks and take steroids while fighting other players who also look like Duke Nukem.
Duke Nukem multiplayer modes revealed [Eurogamer]