Nicraq, you have a good point. And you're onto something based on the literature that you've quoted.
Where this probable "competiton" comes into play for me is that my partner had told me the reason that they opened their relationship was due to sexual issues, she lost interest in him, completely. Yet did have that interst in another. So they opened the relationship.
He does continue, several years later, to reignite that with her. To no avail. She actually flinches if he tries to touch her in a non-platonic manner. Before I witnessed that reaction, I did have a true fear that if they were to regain that, then i was no longer required. And that did scare me, alot.