Because it is wrong.

I am not judging sexual practices. I am judging those who choose to betray their partner. Here is the person he is most committed to in this world, the person who expects undying loyalty in exchange for the same in return, and he is seemingly shitting all over that vow.

If you only care about practicality, then consider that one will not care for the people in general if they cannot care for those closer to them. Honesty and compassion mean nothing to this man.