Having thought this over while I killed crabgrass this afternoon and - while I don't believe in Avon&Cally - I found it a nice mental exercise to try and come up with a way to justify it. (Please note that not quite all of the following is IM-honest-O. But I do think all of it is canonically acceptable if you see them this way, and has been seen this way by other people :-)
There are two (for me) inarguable facts about Avon and Cally's first meeting (Time Squad). This *was* Cally-as-rebel-crusader at her strongest (if somewhat fanatical), and the two of them took one look at each other, and liked what they saw.
I don't agree that Cally would have to go all waif or orphan for a relationship to work - in fact, I'd argue the opposite, even taking into account that Avon *might* consciously prefer a relationship with someone 'safe'. It's very easy to argue - on the on-screen evidence - that Avon's 'weakness', emotionally at least, is actually for the crusader type. Of the three people he's closest to in the crew, two of them - Blake and Cally - *are* the True Believers; whether he wants it or not (and he doesn't, I agree) there appears to be an attraction there to people who can believe in and fight for something outside themselves.
Furthermore, the argument becomes stronger if you can believe that Anna Grant was genuine in her rebellion, and even possibly had leanings that way earlier*. The main problem is, of course, we have next to no knowledge of exactly what the relationship between Avon and Anna was like, so it's impossible to use his actual love affair to build on; but even if there were no signs that Avon could see, it is a remarkable coincidence that the one True Love of His Life turned into yet another rebel :-)
Re waifs, we don't see many, but I don't think he finds them nearly as interesting. He certainly seems to be a little dazzled by Meegat's open worship, but it's all rather innocent - the hand-holding, the gentleness, etc. And could one call Piri a waif type as well? She certainly appeals to *Tarrant's* protective streak, but Avon seems to find her as painful as I do. Veron - a real orphan - gets a brusque "I'm sorry" flung at her before he turns his attention back to Fearless Leader. He is protective towards Dayna - another orphan - but while she may be naive, she's far too Amazonian for waifdom, and the relationship is way too avuncular to base sexual appeal on (that *being* IMO, BTW).
So - based on all this theorising <g> - I might suggest that Avon&Cally could have a chance if she stays strong, a crusader, which means give her something to fight for in 3rd series, so she can turn herself around - but it would have to be something *that Avon would be prepared to fight for too* (the simple and definitely IMO fact is she's not Blake, she's not Anna, he's *not* going to go into it just for her personally. But maybe once he's in there ...)
And as to what that something could possibly be ... there, my friends, you *have* me ...
*I don't, but for the purposes of the argument and based on the fact that I've *seen* it argued ...
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