He just wanted to make sure there wasn't anything leftover, because it would be a rude awakening to realize what that is in the middle of the night when he's actually trying to sleep.

So fine, he sets it off to the side with their shirts - he can wash everything later - and lets Erwin tug him down. He'd had more intention to wait until Erwin fell asleep and then go about his night until he is actually ready for bed...

...but no, this is nice, especially because it's become no secret between them that he loves to bury his face in Erwin's chest. Which he does now, happily, and holds him close.