Because I've seen it so many times before, even on FSTDT before the forums migrated here. No matter whether the younger party was technically above the age of consent or not, if they're under 18, then a story about a teenage girl having sex with an older man will always be slanted as if she was raped, especially if the man was in a position of authority such as a teacher or work superior. This will even be the case if the girl explicitly says she wasn't raped and refuses to cooperate with investigators.
Meanwhile, few people see anything wrong with older women screwing teenage boys. Even if it's not how you would react, I think the general attitude of this thread would be MUCH more condemnatory if it was two male teachers with a female student.