Not sure it's sex alone as such as it might just be openmindedness. For example, if you could get all the sex you wanted out of a partner, but were limited to one or two positions, would it still be as exciting to you after an amount of time has passed?
I really firmly believe in 'variety is the spice of life' and also in the bedroom (or even NOT in the bedroom, go for the other rooms as well).

Plus maybe there are other things being unfufilled, emotional and mental needs.....