You have to wonder if it was really consent, or just the spin put on events. Vivec "sending his head elsewhere" could be a metaphor for catatonia during a traumatic event like [censored]. Maybe the event was more like a prison [censored], Molag Bal overpowering Vivec. Even though Vivec learned the secret syllable "CHIM", it was at a steep cost.
Maybe...Molag Bal put him in a coma, with some dirty love. That dirty love.
(I hope someone gets the reference)