Why are they so spooked? Is the fact that he's clearly worried about her well being not enough to make the assumption that even if there was something sexual going on that it would be consensual? Or is the casual prejudice based off of his appearance strong enough to overwrite that?
Like if I walked in on that scene I would have assumed it was a young couple having some public fun, I might have been a little grossed out but not spooked, nothing implies that he did anything non-consensual to her especially his attitude.