Oh dear, the accident. Some aren't bothered by it, but some are and we have to respect each's position. The "but it's only shit, every one shits" brigade are never helpful with such debates. The fact is, what disgusts an individual disgusts that individual, especially when it's at a visual and olfactory (smell) level.
I live alone so spontaneous anal is something I've never had to contend with. But I hate the idea of mess and the though of the mess was one of the things, apart from the anticipated pain, that put me off trying anal. I got lucky and encountered mess free anal with no pain so then I knew it could be done. When a few tries later the "mess" happened, I was determined never to let it happen again and got myself a few douches. I can douche and be ready in under 5 minutes. It isn't and doesn't need to be a complicated, long-winded affair.
I don't live with anyone so spontaneous anal is out of the question. I always kind of know if I'm going to have anal sex so I have time to douche. Even with douching, there was recently an "accident" as we were experimenting with a new position which went deeper than usual. But he was very mature about it. shrugged it off and filed it under "shit happens" and it hasn't put him off.
If your husband cares enough about getting rid of the psychological block, there is help out there but help you would have to pay for it to be done properly. I wouldn't trust some amateur playing psychologist online looking for guinea pigs so offering to do it free. If your sex life matters, be prepared to invest in it (like some women will pay to get a Brazillian wax or other cosmetic enhancements. Spending money to have better sex isn't thrivalous.
Sadly, the way I see it, he's getting what he needs with vaginal sex. There is no motivation for him to even considering dealing with this unless you start to dig your heels in and make it clear that the vaginal sex will soon cease to be an option.
I firmly believe that if a person doesn't want to do something, no matter how normal that thing they don't want to do may be to the general population, YOUR NEEDS ARE WHAT ARE IMPORTANT. I would feel no better than a trafficked sex slave if I had to tolerate a sexual activity that I would not be doing if I had the choice.
As much as most men make it sound like the woman's needs are as important to him, the truth is, not if those needs means he has to go out of his way to make it happy. You're going out of your way making your vagina available when you'd prefer it the other way. If sex has to be a game of "give and take" then your compatibility is at question here. If "sacrificing" yourself is not a big deal for you, then put up with it.
Have you been with him long? Is it possible to sit down and have a talk with him and let him know how you feel about the anal sex, how much more satisfying it is for you? Then tell him there is help out there to help him get over the accident.
Make a big thing of buying douching equipment. Maybe get him to choose some with you, either in a shop, or online. Let him know you are douching. Make it part of your bed time ritual so he knows you've cleaned out and the chances of it happening again are slim, but can still happen. but if you've douched (and don't over do the douching as that in itself, with water and matter seeping out, can be another gross point. I do a couple, maybe 3 or 4 goes, with the douche gizmo and when its over I always take a sneaky peak when he withdraws and the condom is always clean as a whistle. In fact, cleaner than it would be if it were vaginal.
Sorry this is long, but I hope there is practical, objective advice. If you do not like vaginal sex, you should not be doing it. If he liked anal sex and you wouldn't do it, he wouldn't insist you did it, would he? But if you want to be the sacrificial lamb in the bedroom, then yes, by all means, astra travel out of your body during sex while he has his pleasure and teleport back into the room when he's done.
There is hope as you say he enjoyed the anal while it was happening. But most men always feel that their right to pussy is a given, not an option.
Shit happens as other respondents have said but there are ways of dealing with the occasional turd.
Of course enemas are often recommended but they can be bothersome.
My wife & I have been AO for over 20 years and she has never done an enema beforehand.
She is a nurse so she is not shy about poop.
We have developed a method of dealing with "intruders" as we call them that works well for us and has brought us closer together by incorporating erotic foreplay into anal cleansing.
I may have posted this before but I think it bears repeating.
We start out with my massaging her butt, gently spreading her cheeks and slowly massaging her anus.
Some massage lotion, baby oil or lube is helpful with this stage.
I then slowly insert one finger, gently probing her rectum to feel for any stray fecal matter
The finger is much better able to feel for feces than the dick and it is helpful to start out with a finger rather than trying to push the dick in right away.
If I feel something, and I usually do, she goes straight to the bathroom.
One visit usually takes care of the problem even though she has sat on the pot before we started our session.
Sometimes it takes a couple tries to achieve a complete clean out which only adds to the sexual build up.
I will then check again and if the coast is clear I take the opportunity to stimulate her internally with my finger.
Now here comes the really fun part since I love to help my partner achieve orgasm after orgasm.
Just inside the anal ring, toward the front of the rectum adjacent to the vagina there is a little swelling like the pad of the thumb.
Massaging this with my finger while she stimulates her clit with a vibrator sends her over the edge pretty quickly.
(Did I finally discover the G spot?)
After this initial orgasm she is primed and ready for more.
Sometimes I will insert 2 fingers at this point and bring her to orgasm again and sometimes we just go to straight on anal sex without any worries about a mess occurring.
Hope this helps.
ps We have found that it helps immensely to set the mood beforehand with dim lights, soft music and a bit of "chemical enhancement" - a bit of wine and a few tokes of herb (we live in a state where it is legal).
I think your husband needs to grow up a bit. Nothing in your ass is going to harm him. I'm with analonlylondonebony on this, in that you need to make it clear that vaginal will soon cease to be an option.
Luckily, that happens alone and not with a boy, I do not know how to regain their confidence, but I guess it is talking, it can be uncomfortable but talk to him, I do not recommend enemas because doing them often can be negative for your health.
Talk to him, tell him that you will be careful so that this does not happen, but that you love anal sex very much.
Sex is for both of us to dismantle, not just him, I should understand, to use perservatives as a precaution until I gain more confidence.