About things sucking . . .

My most faithful commenter, Lynda3, commented on my usage of what I thought was a rather clever word - "suckitude," and that made me think of a question that I have had for a long long time.

Perhaps one of my faithful readers can help me answer my question.

I am certain that every single person in the US, and probably even every single person in the entire developed world, has heard the word "suck" used to mean something bad.  For example, the pejorative declaration "you suck," or the ubiquitous declaration "this sucks."   My question is this - what is the item that is being sucked?  I would like to fill in the blank - "you suck - ?"

Obviously genitalia springs to mind, because genitalia *always* springs to mind.  We are, after all, human, the most sex-obsessed creatures on the Earth.  But if so, why is sucking genitalia 'bad?'  Is one claiming that the other person is, essentially a slut?  Declaring, for example, that  - "you suck large amounts of random genitalia" - as opposed to declaring, I suppose - "you suck genitalia that is screened by using at least a rudimentary set of standards for your selection criteria," - which let's face it, is pretty much what everybody does, we just differ on the standards we use.

Or is the "you sucks" pejorative a reference to the old homophobia, declaring, in essence, that the person being slammed is 'gay.'  Saying, to a man - you suck dick,  - or saying, to a woman - you suck the most intimate and beautiful portion of a woman's anatomy?

Or does the phrase imply bestiality?  I know that I have specifically heard the phrase "This sucks donkey dicks" which is not only very nicely alliterative but which also expresses a very strong taboo.

And another possibility - is the phrase scatological, reaching even further back into our development - declaring that one sucks excrement?

I think one could make a case for any of these possibilities, hence the answer remains a mystery to me.

Can anyone help me out here?