In the sixth book of the Christy series,  Susan clearly invites Trip and Wren to join a swinging party without Paul and Christy. Nick did it that way because he wanted Paul and Christy to be able to spend some time with Carter and Kim. However, that meant the events of the party were left completely unknown.

I decided it would be fun to fill in that gap.

At the time, Nick was writing Trip as a character who was very full of himself, so I also thought it would be fun to show Trip not being quite as successful with the ladies as he thought he should be.