Above all else, she already told Chris that she hooked up with 5 other guys before him, that she was going to college soon, and that she lived literally on the other side of the country. Chris stated in the incest call that he felt that any relationship he'd possibly have with Fiona simply wouldn't last long, as he was worried she'd find a long-term sweetheart in college anyway. Didn't really stop him from agreeing to fuck her though. He actually believed this would help her.
Fiona would also claim that Chris was "embarassed and nervous" of the thought of her actually being attracted to him. I'd chaulk it up to him just being lazy/afraid to screw up this once in a blue moon chance of his, since he already blew it with all of the chicks that claimed to also want his dick, but instead turned out to be trolls, hundreds of times.