Here's my guess

In nature there are parasites, fungi, bacteria and viruses capable of turning insects in to "zombies". There's a fungi that infects an ant, causes it to climb to the top of a plant where it clamps on tight until something comes along and eats it. Then it goes through a couple changes in the animal and gets pooped out...where another unsuspecting ant comes across it. There's a virus that infects caterpillars and makes them go to the top of a plant where it somehow causes their bodies to break down in to a goo where it then drips down on to lower leaves where it can infect more caterpillars.

So here's my point. That ant waiting at the top of the plant most likely would have no idea it's behaviors have been taken over...let alone what a bird is when it swoops in to eat it. I think that's what this movie is about...women become infected with some sort of alien life form that effects their behavior in a way that not only protects them from outside influences but even ties up loose ends (via "hiding" their partners) when it's time for the aliens to pick up their cargo. It could be part of a natural cycle of an advanced alien species where they propagate their species or farming some other organic alien life but it's so beyond us we can't comprehend it. Just like an ant can't comprehend a bird or the fungus controlling it.

So basically everyone was just a bunch of stupid ants.