There are always OH- species in water from the self-ionization of water, i.e. 2H20 <--> H30+ and OH-. This adds a bit more. And this isn't a one way reaction. The reactions that require a proton are balanced out by the reactions that donate protons. Maybe not instant to instant but over the long term.

And you need something the balance out the charges. For example,

NADPH and H+ and acceptor <--> NADP+ and reduced acceptor

You need the OH- to charge balance the NADP+ and the H+. So maybe think

NADPH and H+ (and OH-) and acceptor <-->

NADP+ (and OH-) and reduced acceptor

You can't have loose charges running around all over the place-- that would be mayhem.