book about a long-lasting marriage! (and sex)

sorry about the backstory but thought it was important to inform exactly what I’m looking for.

I’m a 25 year old woman – I look forward in my life to getting married, having children, settling down. Not looking for any adventure in this life, just a stable, loving home. But the thing is – I don’t know what that looks like. On some level, through media, (and just generally being alive), I have some semblance of what that is like. However, one HUGE mystery remains: sex.

I often wonder, how does sex evolve in a long-term marriage where no one cheats? What happens to your sex life after 15/20 years of being together, having kids, growing old? What happens at 34? At 45? How does the monotony of sex change overtime? How does the value of sex change overtime when there is lifelong commitment?

It doesn’t (ideally, shouldn’t) be an erotic book at all. I just want to understand the evolution of sex in a loving, monogamous marriage. Through arguments/kids/breaks/compromise/sleeping in the same bed FOREVER – how does it evolve. I’m looking for a book (ideally, fiction – I don’t real non-fiction) that captures that in some way. It doesn’t have to be the central theme, but just touching on it enough for me to understand.