IBS is a strange condition and the nature of that conditions makes your question (and the one in the enema thread) very difficult to answer. You see, when a doctor says 'you have IBS' what s/he is actually saying is 'We have no idea what the hell you have but we do know you don't have: bowel cancer/bowel obstruction/intenstinal or colon bleeding and a whole host of other things that we have tested you for and foind nothing'.

Basically, IBS is 'misc other', you have something wrong with your bowel that causes discomfort but we don't know what it is. Its probably not dangerous though so just take peppermint tablets whenever it flares up.

Which means anything you ask about it may not apply to your particular flavour of IBS as there are likely to be many different conditions which cause it...

My advice would be to try anal sex and see what happens. I doubt there will be any danger from it, it may even make things better in the same way as an enema can. I would advise drinking some peppermint tea before you try it (peppermint is a herb which settles the digestive system and can therefore sometimes treat IBS symptoms so it may prevent any upsets) and paying attention to how you feel afterwards (do you feel better, worse?) Keep a food and sex diary to track IBS symptoms and what you have eaten/done to see if there are any patterns.