It is NTR.
Not "reverse NTR". Reverse NTR don't even means anything.
NTR is not defined by "the girl being stolen".
It is defined by "the loved one" (male or female) being stolen.
Simply, we are more used to see girl being taken away than boy being stolen.
And from what the synopsis say, he is not stolen. He still is with his girlfriend. So, it is more of a cheating scenario.
Well, we will see at the end. But, if the succubus can only interact through his dreams, it can't take away the guy. Theorically...
So, cheating... Well... Can we really call it cheating if it is in dreams? And it is with a succubus, not a human... Personnally, I hardly call it cheating.. xD Because, if we go this way... then, fantasizing about another woman would also be infidelity / cheating... (going on porn game or site would also be considered cheating... ---> well, for the few married one here, what do you think about it? =D What your spouse think about it? Can you ask for us? xD)