A hard "R" vesus a soft "R", perhaps? It just struck me as odd that the existance and level of the violence was acceptable, while the addition of the sex was not. Each to their own? 8-)
Interestingly though, the W brothers didn't follow the normal plot device of exposing the female character more than the male character in a love scene, it seemed that they were careful to treat both charcters equally.
Keep in mind that the violence was not gory -- it simply involved a sizeable body count & LOTS of gunplay. I always attributed the first movie's 'R'-rating to anti-gun attitudes within the MPAA. The sequel, however, is a much more typical R-rated movie.
Anyhow, I think there is a strong cultural difference between violence as entertainment & sex as entertainment. Most rational people don't worry if their kids go around pointing their fingers (or toys guns) at each other and pretending to shoot... but they'd be pretty freaked if the same kids were simulating intercourse.