Why is fucking a fag in front of another painful for the watching fag?? Why would they care?

The punishment is designed for fags that are out of alignment and are thinking too much of themselves and their place. By performing this punishment, the Alpha demonstrates to the watching faggot that it is expendable and not special. 

Even better: when the Alpha fucks a female.

The punishment doesn’t work as well on properly-aligned fags, as they would appreciate their Owner’s virility and acknowledge his right to take whatever he wants.