My impression, for what it's worth, is that it is a relatively recent thing being so common, and that is probably the result of the internet and the easy accessibility of porn. Many will say that anal is a power thing, and for many it may well be. However, I tend to view it is the most intimate thing a girl can give, and so is a form of trust: you aren't going to hurt me, are you. There's no doubt that some SPs are not interested; others definitely love it and cum that way, and I suspect we can all name both types. What intrigues me are the number who used to offer it, and now no longer do. A number who used to work at e.g. Maxes and offered anal there, turned indie and do not offer it. I also suspect, and have no way of proving this at all, that almost all SPs can take anal, but know that it would take time, effort and plenty of practice, opt not to.