In my case, enemas haven't produced an erection for many years. In part, aside from aging, that is likely due to disassociating overt sexual arousal from enema activity over twenty or so years. I did weekly enemas, many coffee retention ones, with the assistance of my wife. She'd cuddle and massage my abdomen during the enema. Not being interested in the erotic aspects of enema play, she was clinical in her approach. Even though we'd often engage in sex afterwards - enhanced on my part by the enema stimulation - the enema wasn't erotic so I "learned" not to become erect.
For the past several years sustained erections require Cialis. Enemas are still just as pleasurable, and they enhance subsequent ejaculation, but they no longer induce those rock hard erections. Even prostate stimulation, such as using an Aneros, while producing a great orgam, will not generate a hardon.