Offering Anduin to do this is not the same as then submitting to it. Turning his back to someone, lowering himself in submission in any way is immediately a repellent thing -- even if that person is Anduin Wrynn. If he were indoors, could sit and let Anduin rise to do it that would be one thing.

Wrathion weighs up his options a long moment, then draws his hair over one shoulder and after another few seconds of hesitation slowly drops to kneel facing him. The tension in doing it is clear, anxiety locking all his muscles and his heart beginning to race. He knows he doesn't have to, knows he could have found another way to do this.

Yet this is a matter of trust, and this is the most open display of it he can offer.