Oral sex after anal sex - is it really dangerous or not?

This topic was quite shocking to me: suddenly I found out that quite many couples practice this.

I.e. a woman does a fellatio just after having an anal sex - and, of course, the special thrill is that it is done without taking care of hygiene (not giving an enema before, and not using a condom).

When I first read about this at forums, I was shocked: this form of sex strongly contradicts with my notion about hygiene :)

But after some googling, I found out that medics insist on the possibility of catching some diseases from the other persons, but that's not applicable in this situation: one cannot infect himself :)

So it looks like the only danger can be from gut organisms, but I didn't manage to find any information on this.

Could you please answer, is it really dangerous, or there is just a psychological barrier, and the danger doesn't exceed other, more traditional, forms of oral sex? (I'll greatly appreciate if you give some links to medical literature, maybe with exact risk percentage).