Believe me...anal penetrating a guy most definently is worth getting the extra, for 2 reasons.
1, if they haven't cleaned themselves properly...you could end up with messy sheets, which warrants an extra fee and
2. Some men love getting fucked more than they like fucking. Its a completely diffferent sensation and unless one's rates are extraordinate, then I dont see an issue with throwing down an extra 20 (or 40)