Porn+Masturbation = Habitual, minimal time spent for low-effort relief, minor enjoyment, 'mood' is not as important, zero intimacy, privacy is important.
Sex = Spontaneous, takes time and effort, more enjoyable, 'mood' is important, lots of intimacy.
Single, couple or married, guys still tend to watch porn from time to time, and as I clarify above, it's not necessarily because the sex life is insufficient, the two just fulfill needs that only sometimes overlap, and stressful working lives can be serious mood killers.