I think it is mostly psychological - the submission, the knowledge that something is going to be done to a private part, which hasn't been touched since her mother tended her there in early childhood, that is arousing. But just as important as the physical process - of the preparation of the equipment, and the knowledge that she is about to share something she wouldn't share with most of her friends even, is the preamble. So it is the idea of agreeing to the procedure, the generalized discussion, the more specific likes and dislikes, and then the developing arousal, suspected or apparent in both the woman, and the administrator, someone in authority - a nurse, doctor, therapist, or matron, which heighten the excitement. Some women may enjoy the regression to an innocent, immature state, yielding to the attentions of a nanny, governess, teacher or tutor, aunt, step-mother, head teacher, or grandparent. That is what interests me, and I suspect many other men and women too.
When I started to administer enemata to willing women, at first I was so delighted - after many years of searching - to find a woman who agreed to let me attend to her, I was so keen to rush on with it, that the scenario became quite clinical, un-erotic and disappointing. Somewhat like "Wham, bam, thank you, ma'am.". Now I'm a lot older, it's the vocal foreplay, the allusions, the double-entendres, the flirting, the reminiscences, the discussions of positions, lubricants, nozzles, tubing etc which are an essential part of the scenario. Also there is often the element of fetishism - white sheets in a clinical setting, rubber or latex items, the colour, the sheen or gloss, the lingerie, corsets, girdles and stockings, nylon, silk, Lycra, satin panties and pillows. And then there are the smells - perfumes, or rubber, or clinical, antiseptic. The preamble should also discuss whether the lady suffers from one of many maladies which would benefit from the administration of a gentle warm slow enema. But maybe she has been naughty, and therefore requires discipline and a suitable punishment, plus, of course whatever additional extras such as massage which may arise during the treatment. Perhaps she simply has an empty feeling - a void which needs filling, a simple feeling of being unfulfilled which needs attention.
As for anatomy, the cervix is often more sensitive than the prostate, and is only five millimeters from the rectal lining. And the anus is a common erogenous zone in both sexes.