To be honest, after you get married, your sex life changes. Period.
Add that to the fact that your W is 50 and you are 35, your sex needs are going to be different. She could be going through menopause or just not wanting it as much as when she was in her 30s. That's just life.
Next, your vague definition of what "good" sex is and how you stayed with partners just for the sex alone.
If all you do is revolve your relationship around sex or expecting sex if you do X,Y,Z, then M isn't for you. People will change and a M is much more than just the sex. Sure it is an important part of it, but what if she couldn't have sex any more? You said that you feel that people shouldn't be in a relationship if sex becomes an obligation. But that's what will happen if she can't perform any more.
If you can't handle that reality, I suggest you cut your losses and find a 20 year old who can give you all the sex you want.
Just my humble opinion.
You're suggesting though that her sex changed from 49-and-a-half to 50. You're also suggesting sex isn't important. To some people, it isn't, and that's ok for them. To me, sex is important. Frankly, I think it's the ultimate thing humans can do; it's better than music, martial arts, or jumping out of airplanes. That doesn't make me a bad person. That doesn't mean I should put up with a sexless marriage. I don't "expect" sex, I WANT sex. I AM rejected, hurt, crushed.
Look, sex is important for me. I'm not alone. I made sure before we married that sex wasn't an outlying obstacle, not as a condition, but simply because the relationship wouldn't have become as strong if it was sexless. Sex helped bring us closer together. It still can. But now she feels pressured to do it, so we're at a road block until she can get past it. The problem is getting her to try to get past it on her own.
