He may've only wanted sex, which would've been morally justifiable
...because while this story ended with a courtship, who's to say this courtship was headed toward an entirely sexual relationship or for a lifelong pairbond (exclusive monogamy)? Some women only want sex and some men only want sex, so who's to presume what the other person wants? Amelie and Nino may well have stayed happily together for the rest of their lives, we'll never know, but because Amelie didn't seek some kind of relationship agreement through an open discussion with him, he could've walked away with a clear conscience straight after sex.
If there is any upset about this, it's probably because exclusive monogamy is the *presumed* agreement.
P.s. Love this film