If he feels that enemas are dirty and doesn't want to give her one because of that feeling, asking him to take one is only going to back him up farther into that belief of his. Too many people think that forcing him or her to submit to an enema is going to work. It won't -- and if you ignore a demand to stop, you're being abusive or even guilty of a sex offense. Arguing with someone about why they don't like enemas is like arguing with someone about their political preferences: arguing only entrenches them deeper and deeper into their beliefs. And, it never works. Especially if you raise your voice or act upset.
May I suggest that if he feels that they are too gross or dirty, maybe the thing to do is to turn him on to how he might feel if he were to try them -- that's not going to happen in one giant leap, but rather by introducing him to one little step at a time.
The indirect approach is what is called for here.
Start with little steps individually and work your way toward the final goal.
See if he will allow you to run a finger around his anus while he is soaped up in the shower with you. If he doesn't push you away, try it again in the shower later that day or at a later date. The time after that, try doing the same with a little lubricant while you're giving him a bj, or during 69. If he still doesn't push you away, put on a latex glove (or finger-cot), lube your finger and after running it around the rim of his anus, push it in very slowly after using just a little bit of pressure on the anus itself. When you can get to massaging his prostate during a bj or 69 (this, still a part of introducing him to the smaller parts of what you like about being given enemas, but in this case, you introduce it indirectly as a part of foreplay) until you can so it by itself -- directly -- as a part of foreplay. (Remember foreplay? It's a team sport!) When you can insert a finger into his rectum and massage his prostate gland while playing with his erect penis, you can approach him about enemas -- either him giving you one, or you giving him one. But you're not going to introduce enemas to him by dragging out a one-gallon bag with 6 attachments and a double balloon rectal catheter. You have to get him to respond positively to the tip of the nozzle by inserting it after you've played with him and the well-lubricated nozzle a bit -- the slow, slight pressure on his well-lubricated anus; in time, you learn how he likes to have his prostate massaged by the nozzle, and later, by the inflated balloon. No doubt you've figured out that he's going to respond more positively by being given 1 pint enema rather than five quarts: see what I'm getting at? Don't be overcome by your exuberance and personal desire...
Don't push too far, too fast. Just see what he will like about what little things turn you on.
Some people on this web site get too rambunctious and want to do everything to everyone instantly! (Actually, I've only read the posts of a few impatient ones... ) The important thing is for you not to try too much, too quickly -- don't give in to your enthusiasm for enemas with the little progress you've made; expect there to be setbacks, as at some time, you can expect his ego to be impacted by fears that liking what you're doing might mean that he's gay. Don't be afraid of the setbacks and deal with them and him patiently: don't forget that women aren't the only ones that appreciate problems being dealt with, with patience, understanding and love -- and don't forget to keep talking with him, especially when he seems to back away. If you're gentle, forgiving and understanding with him (See? Women aren't the only ones who like to be treated this way... ), you're not going to lose him: you're going to succeed in time by breaking your enemas into small steps that he will physically and mentally enjoy if only you have the patience and imagination to attract him.