Really, Nikolai should have expected this reaction. She'd been quite fond of touching him and baring her secrets, so he'd known that her barriers were down and that she was already doing things she wouldn't otherwise. But when she kissed him, something in him - be it either the wolf or some primal human instinct - said yes please, more of this, and he returned it.
For roughly four seconds. Then his common sense kicked in, bursting through the pleasant sensation of her, well, everything. His hands went up to her shoulders and he pushed back from her, shaking his head. "No," he said, you're not thinking straight and... No."
Now, if she'd been sober and had tried this? Well. That would be a completely different story.