Honestly, the amount of effort put in is WAY more. Not really sure how you're not aware of that. Even if the amount of time spent flapping was the same as the amount of time having sex, your effort levels of exertion are dramatically higher. Really not hard to understand.
Also there is 100% a mood thing at play too. One can be in the mood for sex, and one can be in the mood to just not bother. I relate to the guy in this story; work drains you mentally/emotionally/physically depending on the job and that affects your mindspace. Hitting the gym may help or hurt that depending on extra levels of exertion or events that may have happened.
Fapping at that point let's you just kick back and rub one out over whatever. Sex requires not only two people, but at the dame time, at least ONE of them doing the fucking. Unless the girl is okay with doing all the work, it's not likely going to happen.
The ever-going meme of women not being in the mood for sex can apply to men as well. To what extent is to be determined, but this definitely isn't lazy writing.
Not to be mean, but TL;DR- get laid and figure out how sex works relating to effort after a rough day as a whole.
Base +6, Shellstrom +10, nameyguy +6, justforfap14 +6