One of my favorite television quotes is from "Burning Down the House", the first episode of the third season of due South.
"People are not interchangeable like snow mobile parts," says our hero, when his father tells him that one 'Yank' is as good as another.
But, so often in television and movies, women have been interchangeable -- the Bond movies providing a prime example of such a thing. In each movie, there's a 'Bond girl' and Bond's affection for her lasts only through that particular movie, to be switched out for the new girl at the start of the next movie. The Austin Powers movies parody this concept (while, of course, using it themselves).
Many 'action' television series take this same stance -- women flit in and out of the lead (male) characters' lives, only important for as long as they're onscreen, soon to be forgotten (unless the actress returns for a one-shot 'reunion'). Girl of the day, woman of the week, maybe even important enough to make the lead shed a manly tear or two, but rarely important enough to ever be mentioned again.
Action movies (and action/'buddy' shows) tend to assume that their audience is young men, the assumption, perhaps, being that men would prefer to see the hero with a succession of (beautiful, of course) women, as opposed to struggling through a more complex and permanent relationship.
( spoilers for the last Bourne movie and the third series of Doctor Who )