Why are gfs pissed off when their partners masturbate?

Isn't it the most human thing to do. Everyone does it since long before people met their partners, including women.So why is it a big deal when guys masturbate in a relationship. The quantity of sx doesn't decrease, the quality enhances if at all. Porn is an issue, it involves comparison and expectations but why can't you just maaturbate without porn? There are n number of reasons why masturbation is required.