She failed to understand, or accept, that Ichigo didn't want to be her child, but wanted to be her lover. Ichigo was probably unstable from the start, but Sensei rejected her again and again, pushing her over the edge.
What, so her only option was to give in? She clearly knew that that was what Ichigo wanted, but Sensei's still her own person. She has every right to say no.
She didn't have to give in. Just tackle the issue, so that they could live together!
You know what responsible parents would do if they had a teen wanting to assault them sexually all the time? They would seek external help, a psychologist, to introduce a buffer, a neutral third party, that would help everyone in the process of living together. Foster children often have issues and it's not shameful to seek help.
Here, it seems like the signals were there for a long time. As early as when they met the first time! But Sensei obviously didn't want to see them and I think she was actually very selfish.
But most probably, this story doesn't want to be reasonable and the author just wanted to overdramatize the whole thing, because the external world doesn't seem to be relevant.