It's highly implied Nina was sexually harassed/assaulted by elevens before the show, since she is such a nervous wreck as soon as the train she's on crosses into Eleven territory. It's also implied through her reactions to being a hostage by the Japanese Liberation Front, she's a screeching mess when the Elevens put a hand on her, which fuels her obsession with Euphemia when she saves her from their attention.
In some instances, it's proven that people who've experienced sexual trauma resort to more extreme acts of sexual activity to combat the trauma they endured, which is weird obviously but it makes sense with the context of the show.

TL;DR: yurifags are fucking deranged and NOT cute