In my current series, I plan to follow the character's relationship through their lifetimes. I think that as they age, they will struggle to find ways to keep the sexual part of their lives fresh and interesting. It seems fair, because that's what we creators do all the time.
I find that multi-chapter stories will help with identifying with the characters, and also following them as their relationships grow, fizzle out, or stagnate helps to give the story a sense of realism. Otherwise, we'd be better off reading Penthouse forum, or the letters to the editor on the porno magazines.