It's not odd! The arm, the scars, even the mark on your face: they're all you, so it's not odd at all. It's just 'Allen'.

[They're a part of Allen, like how fangs and a tail are a part of Rin. And given Rin's hangups on 'liking someone because of it, not in spite of it' with his own weirdness, he's definitely not going to give Allen grief over anything he has.]