This is the part I never understood, though my wife is the same way. The idea that someone who has not had sex in a couple of weeks, and knows that they will physically enjoy it and that their partner will enjoy it, but still doesn't want it is very confusing to me.
I'm fully aware that there are people with good sex lives. I'm even aware that I could have one - just not with my wife.
There are a lot of things that are important in a marriage. I've decided not to leave because one of them is missing, but that doesn't mean that I am happy.
David Schnarch of the Passionate Marriage and the Crucible book fame has an interesting concept when it comes to sex between married partners.
One of his first controversial concepts is that everything in marriage is a compromise between HD/LD. For example, one half of a couple may be HD chocolate ice cream for dessert every night after dinner. Their spouse could hate chocolate ice cream, but may be willing to have vanilla ice cream once a week. For their marriage to succeed they both don't need to have the same level of desire for chocolate ice cream.
To go back to your not understanding, you can love, really love vanilla ice cream once a week, while your spouse can love chocolate each night and then sneak out during the week at lunch a couple times a week to get a one-scoop chocolate ice cream cone fix. It doesn't make you bad for not wanting chocolate ice cream all the time. It does not make you not love and appreciate vanilla ice cream. It just means the two of you have an HD/LD issue.
Similarly, one partner could have an absolute HD need to watch professional football on TV Sunday, Monday, Thursday and Saturday each week during the season. Their spouse could hate football and make sure (he/she) goes shopping or reads whenever there is a football game on the TV. Again, equal desire to watch football on TV doesn't necessarily make or break a marriage.
The same can be said about sex. Yes, some partners are HD and some are LD and that is just how they are. Sex, desserts, what is watched on TV are all things that a married couple will need to negotiate a compromise on. Most people get way too hung up on the importance of this one element of a marriage. It is important, but as you said in your post, you are still thinking of staying in your marriage.
Now another concept that Schnarch is famous for is that when it comes to sex, you get your partner's "leftovers." That is there are things the will not do and there are the other things that they will do. You get the left over things that they will do that you want to do. So Sex among two married people is the subset of the sex acts I am willing to do and the sex acts my partner is willing to do, nothing more no matter how much you want it.
Sex is important in a marriage, because of the oxytocin bonding hormones and the emotional attachment. If you can really master post coital bonding or aftercare then you develop a killer relationship with you spouse.