seems like people are never willing to accept that if someone who used to love your touch is now disgusted by you, it may be your actions that has pissed her off and is now affecting her ability to be turned on by you. I know when it comes to sex men are more able to separate their feelings with the physical and when it comes to sex it's about the "here and now". But women just aren't like that.
Think about it. If your wife cheated and nags you all the time plus gained weight and dismisses you, would you still be turned on by her? Probably not? Most of those couples have shyt going on behind the scenes. It's naive to think it's always about sex. If something is bothering a woman she cannot get wet, and if the man refuses to even see her point of view, sex is not even a thought. Like
@BlackPearl said, it's mental. There are things that both genders just need to accept about one another and stop trying to put the other in their box. If your partner seems to be turning you down a lot, ask her what's wrong or if there's anything going on in her life.
Older people, married or single, like routine and even dating can feel routine after awhile. You want the same friends, then you start seeing less of them, you become more housebound. Married people are no different. They fall into ruts and get used to the grind. That's life though, you have to make an effort to work around that like you would with anything else.
Also, if you're looking at older women, you need to consider the physical changes women go through with menopause. Your vaginal tissues and hormones literally change as a part of nature. They become thinner, less sensitive to pleasure, and more painful, and it doesn't go away without meds. You have to look at the whole picture and not just blame the woman for being frigid. if your dikk tissue started wearing down and painfully rubbing off with sex, and you weren't able to get an erection, but your wife demanded sex every night, would that make any sense to your body? If she guilted you for your body changes by calling you selfish and frigid, is it your fault? Her sexual desire changes and since it's natural, she may not even think about it until the man brings it up.
Couples will have decide what that means for the relationship, because it's not fair for the man (or woman) to go without sex if they want it for the rest of their lives, but if you're wondering on a factual level why this happens, consider the above.