Nothing behind F’s back (at least no dude behind his back - D has already played with Olivia and Elena behind F’s back).
It is possible D, with F’s knowledge and approval, could have sex in the other room with a male without F being in that room. Not a guarantee, but a possibility.
That would not be NTR since we are narrowly defining NTR to include a “stealing of affections” - see the etymology below:
Netora as a Japanese term for cuckoldry literally translates to "taken away by sleeping with."
Etymology
From
You must be registered to see the links
(ne, “
You must be registered to see the links
”) +
You must be registered to see the links
(toru, “to
You must be registered to see the links
”)