Sforno on Numbers 5:20/ספורנו על במדבר ה׳:כ׳

1ואת, accept it also.

2כי שטית תחת אישך וכי נטמאת, that the curse will apply on account of your having defiled yourself and for no other reason at all. However, if the reason you have to drink this proves to be a false accusation you have nothing to fear from this curse.