Thanatos is not completely oblivious to Gustavain's mental state; he can tell that something is affecting the familiar, though he has no idea what it might be. He doesn't seem to be in distress exactly, so he's not worried about this being a bad idea. (It certainly doesn't feel like one to him.) Maybe he needed a distraction, too, and Thanatos is more than happy to provide.

There are soft noises as the elezen presses in, but they are all clearly pleasured noises and it's well in line with how he looks, near blissful against the flowers. His hands are reached up to hold on to him, hands rubbing absently against the skin and seeming to soak in the warmth of him. Even though they are theoretically out in the open, there's something about it that seems to center them on just them, the expectations of the world (this time or any other) far, far away.