There's a big difference between 'function sexually' and 'getting what she wants from the encounter in itself'. There's a big difference between the prostitute/client relationship and between lovers, just as there's a big difference between an accountant/client relationship and a married couple, one of whom has balanced the books (or better, after they balanced their books together).

A woman's sheer ability to perform can require only an act of will (in some cases), but for it to be part of the basis of a relationship requires a lot more things, which are what they mean by 'chemistry'.