It was a metaphore!!!! (SPOILERS!)
Ok, since every movie is a metaphore nowadays, let me try to interpret this one:
I think it's a political statement, a conservative one.
The five friends are all liberals (smoking weed and stuff, that one guy wearing hipster glasses, etc.) except Kyle. He values the traditional things. When he proposes to April and she says "no", she is destroying the most conservative value of them all: marriage. This monstrous action unleashes the aliens (the conservatives) who are at this point not unfriendly at all... until April shoots one of them. Now the very patient aliens/conservatives have had enough and start getting vicious, they kill and/or abduct everybody except April.
This makes April realize the mistake she has made and she asks the aliens to come back and take her, too (this stands for changing her political views, i.e. "becoming a conservative"). In the alien spaceship she proofs this by finally accepting Kyle's proposal and the aliens/conservatives like this so much that they let them go.
Back down on earth, Kyle and April share a few wonderful moments together, now both conservatives until, a few minutes later, they get shot by "the government" which is, incidentally, LIBERAL right now in America.
So basically, the movie tells us that liberals are evil (they shoot you if you change your views in the end) and conservatives are rather patient (aliens turning evil only after they have been attacked) and not so backwards as one might think (technically advanced aliens).
Right, that's my two cents. I have no idea what I have just written, but message boards say that every weird/bad/illogical movie is a metaphore, so here you go... :D