Warning: Gratuitous Dog-Humping.
The combination of waking up hungover and alone in bed (or beneath one) is probably a sign that the night didn't go your way.
However, these two dogs decided to lighten this guy's spirits and give him the experience he was denied. -- Even if he isn't lucid enough to know it's happening.
Unfortunately, it looks like this inter-species hedonistic fling will exchange crabs for fleas between its participants.