I'm going to add my few cents here...
First, it was Sigmund Freud's Stages of Development that list the "Anal Phase" (age 1-3 years) among others because this was the age where potty training began, primarily with the focus on controlling bowel movement, where and how and when appropriate. Freud's theories were important which is why he remains a central figure in the study of modern psychology, along with those of Carl Jung. But it was Jean Piaget's theories of child development that won greater acceptance. While Freud's theories focused on psychosexual development, Piaget's theories focused on processing and management of information through the child's learning years, and had little to nothing to do with sexual development as did Freud's. I studied psychology for three semesters in college.
As for the discussion on "nerve bundles" in the rectum, I'm not sure where that comes from. We are all born with certain nerve endings in the anal and rectal areas that allows us to sense what is in there, generally so we know when it's time to poop. Any object in the rectum is going to stimulate those nerves which can also stimulate the "pleasure center" of the brain hence the sexual attachment to the anus and rectum. But not everyone has that connection and some people have absolutely no pleasure attachment to that area. And that brings me to the next topic.
I do not believe that enemas at an early age turn us into "klismos" (a word that I dislike). For every person who had enemas as a child and went on to develop a fetish for them, as I have, I'm sure there are thousands more who had enemas as children and never developed the fetish. There are many who went on to have a "normal" attitude toward their bodies and avoided developing a fetish of any kind. I believe that those of us who developed this fetish did so due to that connection between the anus/rectum and the pleasure center of the brain, and probably didn't realize the sexual connection until we were old enough to put two and two together, mostly in our teen years (though sooner for some, later for others).
It was common practice in the 50s and 60s, and certainly before then, to take children's temperature rectally, especially with infants and toddlers, and even older children (the last time my temperature was taken rectally was in the hospital when I had my tonsils removed at age 12). Some kids developed anal fixation when they realized there was some pleasure in having something inserted in their anus and rectum while others did not have that pleasurable sensation. This has nothing to do with the nerve endings in the rectum but, rather, with the connection in the brain. So, how would a mother know if giving her child enemas or taking his/her temperature rectally would stimulate that connection between sensations and pleasure? She wouldn't unless there were certain "signs" such as a young boy getting an erection when seeing the enema equipment or thermometer come out, or a girl asking mommy for an enema when there's no need for it. And, if the mother were to spot the signs, would stopping those practices end the pleasure aspects of anal penetration? Probably not because the child is going to find ways to recreate those pleasurable sensations one way or another. The practices SHOULD stop but that won't likely end the tendency toward anal fixation.
I happened across a YouTube video not too long ago showing a young boy, whose age I would guess to be about 3 or 4, taking a bath with his father. The father was laughing because the boy kept saying he wanted an enema. The father finally told the boy he would "get an enema tomorrow". That's a red flag! In my opinion, the boy was already likely anal fixated and had discovered the connection between anal penetration (e.g. enema nozzle) and pleasure. Should that be the case, it's not likely the boy will outgrow that, but the parents should not encourage it either.