Anal sex is widely accepted in a lot of BDSM play. Men will often enjoy giving anal sex due to the different feel and the fact that the anus is usually a tighter orifice. Additionally, due to the location of the prostate gland, men can be highly stimulated by receiving anal sex by a slave wearing a strap-on or using a sex toy. Although usually more popular with men, women can also enjoy anal sex and the stimulation it provides.
Forced anal sex differs from normal anal sex in that there is the role play of resistance from the slave. Being able to force yourself onto a beautiful, unwilling slave is a very common fantasy.
Masters who are very well endowed should make this known during booking, as some slaves will not have the "experience" to be able to accept them.