It's a hickey, except the boyfriend clearly had no memory of putting those hickeys, meaning the girl fucked the other guy (as you can see, they're both sweating).
My prediction is that either Ouhi will be able to converse directly to Ouna for a brief second before dying again, or both Ouhi and Ouna "die" and they become a new person with shared memories.
Either way, it will end with Ouna or new Ouna completely out of steam and Kazuya will have to have...