Didn't know that, actually. It doesn't seem like they adhered to it much towards the end there, especially at the advent of the Renaissance.
Then again, corruption seemed right during that time, so why would they take a vow seriously?
Honestly, I think it's just completely unreasonable. Most other Christian based religions allow marriage of their pastors and what have you.
I actually found it a little surprising that Jane was a little judgmental of Maura and Daniel when she found out.
It was soooo unlike her.