Sorry, this paragraph has too much going on in it. What I was trying to say is that some people aren't emotionally responsible enough to be monogamous but they think they ought to be because that's what society dictates.
That sentiment was tacked onto the "they" i.e. the people who want to sleep with several people. I may need to edit this by flipping the paragraph in reverse, totally rewrite it or keep this response for clarification.
Thanks for pointing it out!