Of course they are shameful for most people.
Everything is a process.
If you started getting enemas as a young child, you may not have liked them, but it is a matter of course. Being too young and havjng no authority, you accept this, many begrudgingly. This is the first power play. At the same time you are taught not to show yourself beliw your waist, don't lift your dress, show your underwear, or touch yourself, especially in public. What is between your legs is private and you must not touch nor show it to anyone.
Then as you grow, somewhere a long the line modesty kicks in. Maybe at different times as you age. You become aware of things, start putting things together. My first time, I remember having a tinge of modesty around 7.
You may also realize touching yourself between your legs is immensely pleasurable. (You feel to have found this secret that no one knows about. AND your parents are telling/teaching you not to show it or touch it. Hmmm...something is up here. )
You butthole is so close to your vagina for girls and your junk for boys. Girls might be more modest. Getting an enema is still a totally naked butt on the bottom or just panties pulled down. For boys, you are probably getting erections. How embarrassing! A nozzle up your backside gives you a hard-on. What could possiblybe going through that preteen or teen's head? "OMG, I have a hard-on, do they see it, do they know, how embarrassing, am I gay?" You don't know this is a normal bodily reaction.
One can only imagine (as a female) what is going on inside a boys brain.
So now you are a teen. Everything and anything you do that does not conform to your "social norm" with your peers is kept hidden. (Rightly so, you are taught not to talk about such things.) If it's odd or different, if you bring it up you are at best ridiculed, at worst ostracized. Sex is not talked about in polite company. Hell the way people act and talk you'd think no one is having sex.
Everything in life (society) is against you going against the grain. Dancing or marching to your own beat.
Between family norms, society's restraints, religion (which to me is the worst oppressor from which ALL other draconic constraints stem from) the structure in which we grow does everything to squelch any 'off the grid' or fringe thoughts and or behavior.
So I don't necessarily believe (and I could be wrong) that the shame has all to do with SCAT play (although I do find that yucky). Having never explored good clean, safe, careful butt play, lots of (vanilla) people just find regular butt play weird or gross, super straight (vanilla) men think is it gay. (Butt stuff relates to defecation and urination for most so they think it's dirty or wrong.)
A personal anrecdote. I've never wanted kids, and was viewed as an oddity by some of the men I dated a well as people in general. I didn't for the normal routine. I never shied away when people asked how many kids do I have. My answer was always "Zero, and it was the best decision I ever made." That shut them up quick. Some people (that was me) are just not cut out for having children. I just realized it sooner than later.