Given that Christopher was a man, he doesn't believe that Geoffrey's reason for being Emily's sponsor was purely out of goodwill. So, he has come up with his own assumptions. Yet, since even Serena who was the victim was not pursuing it and was even defending them, then it's pointless. Anyhow, no matter, he became curious of Emily.
At first, he was not tempted enough, but later as he wondered more about what Geoffrey could have seen, he noticed her more. Though Emily was beautiful in her own right, her looks still paled in comparison to Serena who looked like a real princess. Was it because Serena has been of true noble birth and that's where their difference lied?