Didn't she hate that bitch?

She wanted to have a sexual experience that involved blood, just like her fantasies
A STATEMENT IS IRRELEVANT UNLESS IT IS WRITTEN IN CAPS LOCK

I can sort of agree with this. Almost everything in her life was out of control, so when she sought out someone to lose her virginity to, the circumstances like when and where, and that she was on her period, was one thing she was actually in control of. Yes, there might have been the blood fantasies too, but I think for her it boiled down to control.
She had also heard those girls talking about how Adam was smaller, and when she discovered that to be true, she said that was fine because it would hurt less. I think that's the reason she intentionally chose him, because again, it was another thing she could control in her life where everything else she had no control over.

There are a lot of different ideas that can answer that question. In all honesty though, I think some things may be looked too hard at.
Losing your virginity is important and a big deal too pretty much every teen everywhere. I don't think it was the "idea" of losing it that was important but more about the "how" she wanted to lose it. I think that was more the idea behind it

Her blood lust did seem highly sexual in her fantasies, so having sex (especially while being on her period) was probably her way of chasing that fantasy.

I wrote that off to her inquisitive nature, her blood-lust as it had to be while she was on her period, and a right of passage.
Fred, Atlanta

I was wondering the same thing. I chalked it up to the weird bloody fantasy she was having during. Just further evidence of her delusions.