Why are the girlfriends in this movie so horrible?

Now instead of bothering to look up the names of the characters in the movie, I'm just gonna say the names of the real life people. Now back on to the point, first off, when Jack Black was killed, they showed Christina Applegate appearing to be bawling her eyes out at his funeral, but I can't help but think it's fake because it doesn't take long for her to hook up with another guy. Next case, Michael J. Fox isn't even killed yet, and Sarah Jessica Parker is already hitting on Pierce Brosnan on screen for Fox to see her, then later when Fox was killed, she is just briefly shocked to see him dead, but clearly isn't too bugged by it because later she shares love talk with Brosnan again just acting like Fox never even existed.

Now I know this is kind of a weird thing to talk about when this is a movie about martians causing worldwide chaos. It's just something I happened to notice.

Anyone care to share their thoughts?