I saw the timing of the proposal as Nick trying to alleviate what he saw a Juliette becoming jealous because of his secrecy. It was his way of letting her know their was no one else. Remember he bought the ring before he even knew their where wesen and Juliette became suspicious. Had he not seen wesen he would have asked her anyway. I have no doubt that she would have said yes without hesitation. I see the only reason she did not say yes was because she knew he had a secrete.
Embrace your inner Biest..... We all have one