Girlfriend/wife/civilian sex is different than SP sex. It can go on all night, for one thing. You can keep playing on and off for a whole evening, while doing other things. Harder to do that with an SP.
SP sex, to me, is glorified porn. Maybe I should say glorious porn. At any rate, I use SP's now in the same way that I would have used porn when I was a starving student--fun way to fulfill a need on the spur of the moment, and a way to expose myself to a greater variety of women.
In calling it glorified porn I am not disparaging it, I love porn, and SP's provide the very best kind of real-life porn experience.