User blog comment:KidVegeta/Announcing My Next Story/@comment-3256091-20101228095529/@comment-3097771-20101229071236

Shinigami are not permitted, nor is it possible to have sexual relations with humans or other shinigami.

That's a rule, but I just typed that from memory so I don't remember which one...

As for the kira to shinigami things, if you note, Light was the only death note owner killed by a shinigami. He was also the only one who (I think) turned into a shinigami, thereafter. -KidVegeta