Ethan had walked in on Jared and Theresa earlier in the day. That night, Jared kept the room they had at the Inn, Ethan and Gwen also got a room there. While Jared briefly left the room, Ethan thought he was going into his room, when in fact, he climbed into bed with Theresa. Thinking they were in bed with their respected others, they made love!
Thanks for posting!