Biology. Looking at an attractive woman actually lifts your mood and makes you feel better. Our cognitive experience is so tied up with our sexualities that we often can’t tell them apart.
The way out of this mess if you can’t for some reason find a woman you’re attracted to is to work on your sexuality. Try to find more things sexy. Try to find aspects about your mates sexy that aren’t necessarily ‘hot ticket’ items, like large breasts and a curvy butt. I think naturally we start losing our standards as we get older.
I do not suggest trying to create relationships with women you aren’t attracted to. This goes double for you ladies. You’re only robbing your partner out of happiness. So much of the enjoyment of sex comes from knowing your partner is into you too.
Don’t lower your standards. Change the way you think about sex. A good tool is erotic literature. I’m a big fan of Literotica.