Come on, they dont... When you are committed truly to someone for life, you tend to overlook such things, and thats what a marital relationship is all about. I do agree if you are physically fit, you will be better in the 'art' but that doesn't mean your spouse will love you more (or less).
The art of love making is a completely different subject. Love-making is not just a physical thing there is a lot more to it than that.
Loving someone out of compulsion or simply going through the motions and actually wanting to make love to someone because you find that person attract are two different things. There is a lot more fun and excitement in the latter, making your quality of sex life that much better.