I mean that as soon as the sex scene comes up on the telly, protocol dictates that one must avert one's gaze, get up and mutter ''I'll just put the kettle on'' and proceed so to do. If executed properly, it should seem entirely coincidental that you chose that specific moment to go and make the tea.
If you come back in before the sex has ended, then you can easily retreat again with the excuse that you forgot the biscuits.