Assuming they are not actors, I'd reckon they at least sign some form of release and know what they are getting into. As for sexual orientation, women are less attached to their sexual identity than men are, so when they get aroused it's easier to accept things out of the ordinary.
There are so many lesbian massage videos from Japan, a lot that seem to feature women who are genuinely not actors or acting... is that right? Is it that many Japanese women are open to lesbian experiences, even though they identify as straight?
Those sub titles are basically my usual chat up lines ! It's a lesbian massage so they don't use the one about 'swallow this the protein is good for you' :-)