To answer the original question, is constipation the only legitimate reason for an enema, I’d ask,“legitimate in whose eyes?”
I think the sorts of enemas we take are viewed as fringe, last resort, or potentially dangerous by most clinicians now, so arguably they are not even considered legitimate for all but the most serious constipation/obstipation.
Speaking personally, I see lots of reasons I consider legitimate to take an enema. These certainly include constipation, but I don’t want it to get to the point of legitimacy within current medical practice. My body reacts extremely poorly to laxatives, and my most common IBS symptom is pain, with or without getting notably blocked up. For me, enemas are a way to avoid or shorten flares, and I always try to use them before things get too bad.
I also see enemas as a good way to rehydrate and rest when sick with a respiratory bug, and I’ve had luck with them helping considerably during sinus headaches. (I have not tried them for migraines because, oddly, I am hypersensitive to the sound of running water during a migraine. I may be the only woman on earth who sometimes doesn’t want to hear her spouse doing dishes!)
I think enemas have legitimately improved my skin and mood on too many occasions not to have a causal association, but for me it’s too much bother to resort to an enema for those reasons alone. For other people it might not be. Same thing for the enema as an aphrodisiac; they definitely bring lots of blood flow to sensitive areas and heighten sensations, but I’m some combination of too lazy and repressed to use them that way, and tend to seek them out only when in a degree or more of medical need. (But you can see from some of my other posts that after my medical issues are helped, I find the process sensuous and can be ready for a very good time.)
So—I think enemas have a bad or limited reputation in society generally, but a very good and useful reputation that extends to and beyond constipation relief for me. I don’t know that others without our experience or proclivities would agree or view these as legitimate, though.
Last, if I had to start delving into the deeper “why” of their appeal for me, I think an enema from another person is an expression of very deep and unconditional regard, the sort of loving care that you are unlikely to find in life from any but a parent or close relative. It isn’t just a medical procedure but instead is a trusting surrender on the part of the recipient and is a great gift of acceptance, patience, and attention from the giver. If you are lucky enough to find and enjoy this as an adult, it’s hard to find a deeper act of connection.