[He's a little amused so there's a small smile.]

Of course. [And then a small pause as his countenance changes to something a little more serious.] But I swear to you that I will never hurt her and do everything in my power to treasure and protect the young maiden...Chibiusa's heart.

[Well if that didn't sound like some kind of vow or proposal, can only imagine what he would say for the real thing.

And he even referred to Chibiusa by her name! He's definitely being serious, even if he knows he doesn't need to.]