Some of us have wondered since PoA how Dementors reproduce. There was a theory that had some following which suggested that it's possible those who recieve the Dementor's Kiss become Dementors themselves. Although of course there is nothing to back this up in the books. It was a rather horrifying idea, though.
I think it's not the idea that they are breeding that is gross. What makes it gross is that their breeding creates a freaky mist over the entire country. What is Gods name could possible cause that??? I really don't even think I want to know.