This is only my opinion, and I can only base it off my own experiences.
Anal sex is a huge bugaboo for me in a lot of ways. My husband (who is also owns me) knows this. It excites him, for a lot of reasons- and, sincerely, most of them not selfish reasons.
We do have anal sex and anal sex-play. For the most part, it thrills me. I sometimes love it and sometimes hate it, but it never fails to thrill in some manner. He has not thus far used it as punishment, though there have been implications that it could be used as such, but not to my detriment. I do not know how often other people have anal sex, but I'm quite sure other people do.
I cannot think of even one sweeping statement that I'd be able to say allMasters/Mistresses demand from the one(s) they own. This lifestyle is highly personalized. So I do not believe all Masters would require anal sex.
Your last question... "Should subs submit..." is one that I will give a rather standardized answer to any question that begins that way: If it is actually detrimental to the sub's well being (mental, physical, emotional, etc), absolutely not. Here's why you should draw that line: If it's causing you damage and you are not communicating that, then you are damaging his/her most precious property.
If it is just uncomfortable or not thrilling or the sub isn't getting anything out of it, then that depends wildly on the nature of your relationship and is probably (again, depending on the nature of the relationship) something that should be discussed between the two.
I know, that's not the sexy answer, but it's how I've found things work out in my own life.
I have to respectfully disagree with likening anal sex as punishment to rape, at least the way it was phrased above. To me, that is akin to likening spanking to abuse. Unless the anal sex is damaging and the sub has made it clear that for whatever reason, he/she cannot handle it (ie; it's a limit, hard or soft), I think it is a completely valid punishment. I say that with the understanding that punishment must be handled correctly- it must be for the right reasons. If the submissive is ultimately harmed on a basic level, not fortified, by the punishment, then whatever it is, it can easily become abuse. To me, though, it's all about communication. Unless I tell my husband (via safeword or otherwise), I don't expect him to distinguish my "No! No! No! nonono! Stop!" that just comes out naturally, from my actual need for him to stop. Does that make sense?