The ultimate relationship: friendship, companionship, love, lust, sex. All together with the same person.
Many of us can't find that person that will give us all of that at the same time, or that will last a lifetime for those who are married or quasi-married.
The next best thing is to get the missing attribute separately from different persons if you're unable to find everything in one person.
Sex is where the SP comes in. It can be mechanical, I agree, but I've told SP's in the past that she's my girlfriend for an hour, and try to treat her that way. There is no expectation or obligation that she reciprocate, but your own effort usually merits some in return.
Meanwhile, you can appreciate someone else's company, even if sex won't ever happen for whatever reason. But you'll have to get your rocks off somewhere else.
That's what keeps me satisfied..... or at least sane.