Well open sex could be a number of things.
As someone mentioned it could be as in an open relationship, where couples have an agreement to see other people for sex outside of relationship, but without any emotional entanglement with those other people.
Or it could be as in having sex in a public place, like a train, plane, park or even dogging, where couples drive to secluded places, with the knowldege that others are in the bushes watching or taking photos.
It could even mean, again as suggested swingers parties or clubs. Many men fantasise about attending such events where couples either have sex together in full view of others or have sex with other couples or even engage in full blown orgies.
It sounds like he's encrypted the file, so only he can view it. If it was me, no matter how, I'd get into it to find out what he's up to.