It's b/c women in relationships generally value 100 things before sex. If I were to write down the top 25 things I want in a man his sex game wouldn't even make the list. So if the sex isn't the best and everything else is good we can deal with it.
I can't speak on other situations but for me it didn't start off terrible. It was fine at first but a few things went wrong. First, he wanted to have sex like 5 times a day. That was fine (well it's not) but I am kinda a creep. I get off on "excitement". I don't want sex every day. I want the build up. The "best" man I ever been with had a similar preference. We would have sex once a week for hours and it was amazing. So the initial excitement of having a new partner went away faster than it normally would. So b/c I was so easily excitable before he figured he didn't have to put in as much "work". So when I didn't get excited as quickly he took it personal, got frustrated and impatient.
Second, we wasn't willing to put in the extra "work". Again he took it as a hit to his pride so he didn't want to do it. I communicated what I wanted a million times and just endured. He would usually do what I asked once then go back to the same bullshyt. On top of that he was huge so it just made it 100 times worse. It got to a point that I just hated having sex with him.
But I never pretended I liked it when I didn't. I am sure that added to the frustration. Women fake it a lot. That's why so man many think they are so amazing. Lol But I didn't. I am too old for that shyt. I am fairly easy to please. I just need 5 minutes of physical touching and I'm good.
Eventually we stopped having sex. I was tired of being ignored. We got into a huge argument about it and I told him I am not having sex unless I enjoy it. He got in his feelings and try to come at me with "well other women are okay with xyz and don't need xyz" which I guess is the equivalent to the the bigger dikk comment. shyt didn't phase me because I have never had another man take issue with foreplay. I told him he was lazy and the dikk was trash and I never wanted to have sex with him again. And I never did.
The end.