Hullo Ivy! I find your story filled with suspense (what is going to happen with Ella and Ben?), so I can't help but to comment now because all of my assumptions might turn out wrong.
But yeah, this:
Originally Posted by Ivy
First, Ella's ability to garner male attention was astounding. Much of the time, she expressed no attraction toward these men, or even a bit of disgust, but would still cultivate their interest. The problem was, she seemed utterly unaware of others' feelings, and treated sex as something entirely meaningless.
Case in point: we went out for drinks at a bar. Within a couple hours, three men had bought her drinks. One of them stopped paying attention to a woman who was clearly interested in him (not sure if it was his date), in order to gain Ella's attention. She started idly stroking his arm, and the other woman left, visibly upset. Another man bought her a second round. She told me she found him extremely unappealing, but by the end of the evening was asking how I'd feel if she went home with him, instead of me--she felt obligated, since he'd bought her two drinks.
I had a massive emotional conflict that night. First, I was envious. I had never had even one man, save Vino, buy me a drink without disclaiming it as "just friends." Second, I was worried as hell--is this why she had been sexually involved with that many men? Because of a sense of obligation? If so, why was she involved with me? Finally, I was pissed--she almost left our date to go home with a man she didn't even like!
To me, the way you describe Ella seems like a classic case of a woman who has been sexually abused. She feels like she has absolutely no control over her own sexuality, and feels that it is her fault if she is abused/pressed into doing things she doesn't want to. Because she so clearly is unable to stand up for herself, she involuntarily sends out all the wrong signals to all the wrong men.
Her weirdness over Ben having any other relationships might be just that Ben is the only man in a long line who has respected her as a human being, and she is afraid of losing that. Since it appears you also treat her as something more than a fuck-doll, she might simply have experienced a double-whammy of insecurity with the thought of you two becoming involved and inevitably tossing her in the process.
Another point that struck me was that your interactions with Ben and Ella all seem to revolve around heavy drinking.
Anyway, really excited to read what happened next!