The purpose of the filming?
Part of me wonders how people regard the filming. Is it part of the sex act? While there is merit to the argument that no, it isn't, what is the covert filming actually for?
Additionally, there is some blurring of law and social processes. Speaking in terms of psychology, for instance, one can reasonably assert that the voyeur in the park, or filming in your windows, or whatever, is, indeed, a participant in a sex act that at least one partner did not consent to. In terms of the law, it generally falls under a different purview than rape.
However, if the filming is part of the sex act, what are the implications? To take Orleander's point, for instance: If you consent to sex, does that mean your husband can flip you over and plow your ass?
If you consent to give oral sex, does that mean you have inherently consented to vaginal or anal sex? If you kiss a guy on New Year's, does that mean he can knock you down and have sex with you?
What, exactly, is the purpose of the covert filming?