Could you explain two 'sxxx happens' jokes that I can't understand?

I found 'sxxx happens' jokes about religious concepts on the internet. But English is not my first language.So I can't understand two of them. First of all, it's not my intention to desecrate sacred concepts. I'm not angry with these jokes. I just want to know what these jokes mean because I like jokes.

Here is the link and jokes I can't understand;

http://www.urbandictionary.com/define.ph...

(This link doesn't work here.So if you want to see all jokes, reseach 'sxxx happens' on www.urbandictionary.com)

A universal religious concept:

Krishnaism: Sxxx happens, ring a dingy ding.

Mormonism: Knock Knock, Sxxx Happens.

What does 'ring a dingy ding' mean?Does this have a relationship with an album of Frank Sinatra?

http://en.wikipedia.org/wiki/Ring-A-Ding...

Why does the author of these jokes use a knock knock joke? Is it nothing unusual to see house-going Mormon missionaries in America?

Could you explain two 'sxxx happens' jokes that I can't understand?-Myspace pictures

The "ring a dingy ding" is referring to the fact that Hare Krishnas often stand around playing cymbals. It has nothing to do with Sinatra.

You understood the Mormon one. Their missionaries go canvassing from door to door.

Could you explain two 'sxxx happens' jokes that I can't understand?

-(Myspace images myspace.com)

Sounds like the overall joke is that sex, which is often negatively portrayed by varying religions, is a universal religious concept.