People say it's an old part of western culture but I'm not so sure of that.
I think it's a consequence of MPAA ratings (especially after the 80's with PG-13 which guarentees violence but no boobs e.g. MIB3) and movies going for low ratings in order to get bigger target groups. Also the rise of neo-conservatism or whatever.
I was watching Police Academy (1984) and it had plenty of boobs but I'm not sure an R rated movie would use so much boobage unless it was a gimmick today.
I'm probably more annoyed about boobs as some gimmick (Ubisoft E3 conference) than them not being there at all but some of the gore felt over the top at E3 this year. People who think desire stems from some outside influence puzzle me since it's just something human beings (I can only speak for men) are "programmed" to have. All in all it's probably some knee jerk reaction because sex isn't against the law but civil violence is against the law so we call that unrealistic.