Not really that hard to figure out at all.
Two thoughts: Remember that line, "no matter how great she looks somebody is tired of fucking her". Which usually means if it was just the fucking it would be okay, but with the fucking also comes the person and all their attitudes, behavior, and baggage. Which leads me to the next thought that we know as a truism: Sex is in the brain. Moral of story: No matter how great the fucking, it can only be physical for so long before the personal aspects seep into the relationship. Those end up being the most important determinants of whether good sex continues, or not.
Just my fucking $.02...........