What are the differences between sex cults and the tantric tradition?

What is the difference between sex cults, and the tantric tradition? I understand that the latter has a lot of tradition and ritual behind it And while there is a strict monastic code, I imagine, in the tantric tradition, monks are encouraged to have sex with their disciples. Furthermore, any claims that there is a direct lineage all the way back to our Buddha, Shakyamuni, is as doubtful as it is in zen; these practices may be Indian, but aren't specifically his. I'm not saying that a power imbalance in a religious context of sexual activity needs to be abusive, but it can be. So why not here?