I am SO sorry. I know how you feel. I have the same. I NEVER talk about it except to my partner and sister, but I have some advice if you're interested. I don't know how severe yours is, but mine came along with endometriosis several years ago and I got sick as heck! I just kept getting sicker and sicker and eventually only weighed 70 pounds. It's been a miserable battle and the endo doesn't help. I am only 21 and had endo since I was a teen and 5 years ago started having the severe IBS. I finally had surgery this year and was put on meds for it which worked like a miracle. I used to not leave the house. I couldn't do anything. Now I've totally gotten over it. It's a miserable thing to have. Unfortunately, I never have gotten into anal sex. My partner and I don't do it, but I do have some wonderful suggestions for you if you want them.
I guess a good idea would be to make sure to flush everything out before hand. Ive never done anal sex like I said, so I don't know much. But some people have IBS C or IBS D one is constipation and one diarrhea. I never had the bad diarrhea problem and not horrible constipation either, but before using an anal douche or enema be careful. You may not even need one.
If constipation is an issue, you may consider flushing out first. But having severe IBS can also mean severe pain when you use enemas or anything like it. I never have used enemas, but even using drinks that clean you out or pills to do it would cause flare ups for me. I would suggest using a natural lemon mix to do that if you're worried about those things before anal sex. You can make the lemon drink at home.
I wish I had better advice, but the only thing I can advise is how to clear up the IBS. I was determined to get rid of it and I was suffering every single day. Now, I've been using different things and finally found a solution. I started using virgin coconut oil which can totally reverse IBS (IT DID!) AND I used aloe vera pills. I've not had a single upset for months! I finally have a life! If you are interested in the details I would be glad to discuss them with you. Message me anytime.
You could try peppermint oil to relax your intestines before hand, but I've tried absolutely every thing for the pain of IBS and with no luck until I started a coconut oil habit. It didn't even take a month to rid it and I suffered for years.