Wouldn't they think that the MC (being the rich bugger he is) would find out? They might not know he is an assassin, but they do know he is rich.
If he cares about having sex with her, he wouldn't allow her to get defiled, or rather would defile her himself.
If he doesn't care about having sex with her, he'd generally care about her mental wellbeing.
Either case it's still bad for her to be defiled right before being adopted. The rich guy could complain to the authorities about the orphanage which would generate unnecessary trouble for them as their skirting the law as it is already.