It will put you at higher risk for any disease where oral-fecal contact is the vector, so anything from hepatitis to cholera to giardia, say, plus of course the usual parasites. I don't think there's really any specific biological risk aside from the fact that there are some illnesses that are transmitted butt-to-mouth (I don't think it's a severe risk factor)
(And anal sex remains less frequent than you'd guess from its prominence in porn and how often it is discussed.) The rate of (non IV drug using) heterosexual HIV infection in the US is very low overall, so even if people are having somewhat riskier sex it isn't going to make that number jump immediately. Has there been an increase in heterosexual HIV transmission? Hepatitis? I suspect the effect is small and hard to measure.
