Why'd he let Persephone in? Well that was certainly a good question. If he could get his hands a time turner he'd definitely redo that little moment. But at one point he'd trusted Persephone and it was hard to just move past that. He hadn't really thought she'd try anything as long as he let her in to talk. Then again, Marcus could be an idiot when it came to girls. Might as well be honest with his answer. "Because there was a very good chance that if she stayed in the hall, I'd end up hexing her. And it would have been better to happen in here where there wouldn't be any witnesses," he said with a shrug. Plus, he didn't need anyone reporting his actions against the bint to the Ministry. His yearly check in they forced him to do was bad enough without giving them reason to add on to it.
"I didn't honestly think she'd be stupid enough to try anything like she did," he said honestly. He knew Persephone was capable of some awful things, but she'd stopped trying to get away with them with Marcus long ago. However, as Katie continued to speak, the thought of him having realized that he not only didn't love Katie but was suddenly back in love with Persephone was just absurd. "What I felt for her was never love, Katie. Don't believe everything a child says. You can't love something that's soulless." Of course, he'd thought it was love at the time, but he'd quickly learned his lesson about that.
When Katie asked him how he would have reacted if the situation was reversed, he took a moment to really think about it. He didn't want to just feed her an answer. "Of course I would have some doubt. But after cursing him, I would have at least given you a chance to defend yourself before completely flying off the bloody handle at you. I know you Gryffindors are prone to just charging ahead and acting and waiting to gather details until later, but fuck, Katie," he finished, beyond frustrated at her. "We may be a shite match but that doesn't mean that every little thing between us has to be bad."