I guess that depends on everyone's beliefs. I see guys watching porn while they're in a committed relationship with somebody else as a huge way of disrespecting not only his relationship but the girl they're with at the time.
They're still looking lustfully at other women. I don't like it at all and I think it is insensitive. I just don't buy that whole "they have needs" excuse and the whole "poor him, he wasn't getting any so he had to relieve the 'pain' somehow", as Lucas said. I think love should be greater than that and the fact you can control your so called "needs" is just proof enough you love someone and are able to put that person first instead of yourself and your own desires. If you truly love someone, that someone should be enough, for everything.
I guess to me it's just a question of maturity and guys not thinking with their lower heads, which is something almost impossible for the most of them, I know.