Look, I'm not arguing that forcing sex is wrong and that we have the right as individuals to refuse. We're on the same page.
But, I can tell you that there's sometimes a dynamic between a married couple, when sex is viewed as something given to one another freely. The "fight" may not be a fight in the sense that you're imagining. The sex is still consentual.
I didn't love my ex husband. And it was very hard having sex with him for that reason. He often bugged me for it and I often didn't want it but would give in. NEVER did I liken this to rape, as I was his wife. It was never done forcefully. It was more of an annoyance when he bugged me.
The "fight" wasn't an objection in the sense that I was being violated or feared being violated. We had a certain comfort level with each other to where that just wasn't an issue. And though there were deeper issues as to why I didn't enjoy sex with him, my reasons for not wanting sex were usually quite practical - I was exhausted - I had to get up too early the next morning.
Perhaps I can look at this from another perspective because I'm far from being a push over. I won't give it if it's something that I'm not comfortable with. But, I totally get the dynamic of a married couple and their trust. You may not be in the mood, but when there's love and trust (or an assemblance of it, as was the case with my ex husband), rape would never cross your mind unless that trust was broken and something violent happened.