Much like New Car Scent when you have a new car, New Relationship Scent is that glow of being in a new relationship. You're more caught in the excitement of getting to know the person than actually getting to know the person.
Guy 1: I met a really cute girl the other. She's amazing!
Guy 2: Finally getting some?
Guy 1: Not yet, I want to make sure it's not just New Relationship Scent or just a random hook up