Ii can also agree that seeing someon have sex is not inherently sinful. But we are walking a very fine line here.
Seeing someone have sex accidentaly is not what we are talking about. Anytime sexuality is taken out of the real marital context and represented as an object for pleasure, that is sinful. That includes a couple making a video for their own use.
I know this seems harsh to a lot people but it is not. This teaching protects the sanctity of sexuality and the dignity of people. A representation of sexuality, used for pleasure, makes the participants, and the sex, an object. Sex is not an object, it is the fruit of the sacrificial love between two people. It is the fruit, or the fulfillment of, a spiritual and physical reality, not a representation of it.
The pleasure involved is a beautifl thing, but it is not an end. So anything that is done merely for pleasure is objectification. It is not foreplay. Foreplay is different. Foreplay is part of the act between two people that ends in mutual self donation. The Church does not prohibit foreplay, although certain acts are arguable.
But there is a difference between an object, or representation of something, and an act between two married people.