why does it always have to be the girls that are possesd in these movies

Because girls are vulnerable to evil? And Satan is a creep!

Ash nazg durbatulûk, ash nazg gimbatul, ash nazg thrakatulûk, agh burzum-ishi krimpatul.

Purely so they can be a sex object I think, as in most movies. Good for the trailer etc, a sweaty girl lying on a bed with her legs akimbo etc...

One girl and one man are possessed in this film, and one boy suffers extreme demonic harassment. The better question might be, why is it that male suffering is invisible to you? You sound like the sort of callous person who would nod along as Hillary Clinton says "Women and children are the primary casualties of war" - completely ignoring all of the dead men who fought in the war.

I'm concerned for the state of your soul. You lack compassion, meting it out based upon group demographics, and politically correct standards. This is evil. Instead of thinking of yourself as a victim, perhaps you should focus on how you can help others around you.

There does seem to be a tradition regarding girls, around the age of puberty or just after, being the most common targets? victims?... something. Not just possessions, though... various other sorts of paranormal occurrences as well.
I suppose part of it might be due to male attitudes towards women 'coming of age'... but there might be more.
Either way I don't think it's because women are seen as weaker, more like they're seen as desirable... fascinating to the men telling those stories.
I'll just say that in my own experiences I've heard FAR more stories about weird goings on from women than from men... and often the stories take place around that age... younger teens and such. Ghost stories, UFOs, stories about strange pets, imaginary friends becoming less imaginary, psychic phenomena, seeing 'angels'... lots of stuff that I hardly ever hear from male friends.

Because it is easier to sell a film about a girl writhing about on a bed and tearing off her clothes than it is to sell one about a sweaty teenage boy.

Atheism is a religion in the same way that celibacy is a sexual position