Normally I hate this about anime, but it actually makes sense here. The MC clearly only wants a pure, untouched, virgin girl, at least at first. He legitimately believed that all the censorship was good at the beginning, remember. Once he starts believing in what they're doing, that's when he comes around and starts liking the other girl despite her perviness.
Imagine meeting your waifu, only to find out that she was getting fucked by chads ten minutes before talking to you. It'd be like that with the MC and Anna.