I would echo what Alison said from a guy's perspective. I have struggled with much of the same types of issues in my own MR. I had a couple of rules: if she ever said no, that was the end of it. (That doesn't mean I didn't pout or whine or complain or get resentful, which also was not good, but I didn't PRESS her on it in the moment.) If she reluctantly said ok, but I sensed any sort of resistance, I would not continue either. (Again, not that I handled it well afterward.)
I am not sure if Alison is going at this from a legal standpoint, I certainly am not. I think there is a huge difference LEGALLY with a W doing her "wifely duties" out of obligation versus rape. I know some that tend to lean a little left on this topic and probably take a harder stand, which I would disagree with.
However, from an emotional standpoint this is VERY VERY important. As Alison said "Love is patient and kind. Love is not about you getting what you want out of someone who does not want to give it." Your W has to feel safe with you. Pressing her on it and her "giving in" might not have legal ramifications (and shouldn't), but emotionally she could put you in the category of a sexual abuser. And that obviously could have detrimental impacts on your relationship with her.
Further, guy to guy. Is that really that satisfying? I mean, at that point masturbation would be just as "satisfying". Not to get to explicit, but if she isn't into it she just becomes a hole to you anyway.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018