If teenage girls were all like Bella Swan, it would be 1708.

Or, Why I won't be letting my daughter read Twilight at 13.

I read the Twilight books. I actually read the first two before books three and four were even out. I was ahead of the curve on this one, since my husband's high school students gave him the heads up. So, as a favour to him, I read the books, which he duly stocked in his classroom. At least, he did the first two.
He may add the third to the collection. He will likely never add the fourth.
It's not just the sex (and if that was a spoiler for you, tough). It's the attitude towards sex.
That Stephanie Meyer treated sex as something that happens after marriage in the first two books was something cute and sweet. That sex (and marriage) becomes entangled in ideas of predestination, fate and inevitability, rather than just love, is a problem for me.
Take, for instance, the werewolves and their imprinting. The werewolf meets the right mate and the two fall in love, no matter what. Meyer, early on, in an unimportant character, introduces the idea that a werewolf, who will age slowly because of the change, will imprint on a baby, if that baby is the right person, and will duly maintain a protective, rather than romantic or sexual, relationship with the child until the child is of age.
There's something so sick wrapped in that harmless idea, that someone will love you your whole life, acting as an uncle or a close family friend might, and then taking that relationship somewhere else. Is it Mormon idea? I don't know. But I do know that I don't like it. I don't like the idea that girls will read this and think it's okay to try to cultivate a romantic relationship with an adult close to their family. Adults attempting inappropriately romantic relationship with family friend's children are abusers.
It's not my only issue, but Jacob imprinting on newborn (though growing fast) Renesmee, the vampire-human hybrid, is a definite problem for me.
Another problem is that Bella gets married and throws herself completely into her relationship with Edward, to the exclusion of our family and friends. Yes, it's because she's a vampire, but is that really an example we want our own teenage girls to have. This heady, romantic, sexual relationship, as typical in intensity as any first love/lust, is treated as something that can be taken as an excuse to escape, to become someone new. Putting all ones eggs in a basket - and the wrong basket at that. High school graduation does not make one an adult, and going to college is not a light experience that one can commit to half-heartedly.
In the end, I hate the be-all-and-end-all Meyer gives married life for Bella. It changes everything for her in irrepairable ways.
At least she hit that nail on the head.

This comic by Lucy Knisley sums up the ridiculousness of the books nicely. They are a black hole that sucks you in and it's nigh impossible to think critically of them while caught in their thrall. After, it's much easier to see that they have all the substance and nutritional value of a bag of chips, with the same quality that lures you to keep eating, without thinking.