Plus, forgetting the sexuality stuff, I would say most teenagers don't have that kind of sex life at such young age, so it's even less probable.
It's not typical, but a high level of sexual activity isn't so rare as to stretch plausibility. I knew people in high school who were definitely getting it on a lot. And this is an idol band, they're selected for being cute and attractive.
The entire band being girls who like girls is a bigger a stretch, or would be if they were thrown together at random[1], but queer people sometimes clump up socially, so if it was a group of friends then they might be pre-selected for that too.
[1] Even if we take 50% of women as being bisexual/lesbian, 4 of 4 being bi would have a 1/16th chance. If 25% are bi or lesbian, less than 1% chance.