My interp was that Diane realized she had lost Coop (day suddenly turned to night). New Mr. C wants sex so she complies because she is really hoping that someday her actual Coop will return. 

It could be problematic because it conflicts with the narrative she gave Tammy, Cole, and Albert as a tulpa right before they shot her. I don't really want to get into it, but it was bothering me as a choice that Lynch made.

Posted by: Jank Frones

Does it have anything to do with Diane and Cooper's awkward sex scene?  Neither seemed to be doing it for fun, but rather for some other purpose.  It seemed like they knew they had to do it to fulfill some purpose - when Cooper says, "now you come here to me", like he's following instructions he got from who-knows-where.  Who knows?  Hopefully the forum has some ideas...

I think that they were performing a ritual to release an energy that would allow them to cross to the other place. 

Remember the shadow appeared when the couple were having sex by the box, so something like a portal must get opened.