This is the grey area aspect of affection vs. sexual assault.
I'd love it if I woke to my partner going down on me, despite never having said it, but in a shorter relationship, hell no!
I guess the strict definition is 'consent'. You can't give it while asleep, or even say "you may wake me by doing that", because you cant consent to it during the act.
But then the other side, is a boyfriend reaching around you while you sleep (spooning) with his hands on your breasts, 'assault'? A husband? Where is the strict line? Maybe the line for this is decided by the woman in the moment. "He kissed me there, I felt violated because I was asleep" vs another woman saying "I woke up and found that really cute"...
Honestly (personal opinion), in this case, I think he intended it to be cute and romantic. The fact that you were naked has certain implications (perhaps already had sex, comfortable with him touching you, etc). So I think kissing your bum, despite being a bit weird, falls into forgivable-to-grey area realm.