She finds the answer... Complicated, and she can't answer his follow ups.

"Well... I've never thought about it... I would say a sin is whatever God is against," Asia replied.

"Yes, but for what reasons should he be against certain things?" Issei asked her, making her pause again. "****, definitely. That's something harmful. Theft? Also a definite sin, though that one can be understandable in circumstances like if the alternative is starving to ****. But what about things like being a Devil, but not hurting anyone unless one has to? Why is simply being a Devil bad?"

Asia opened her mouth to answer... And found she couldn't. There just wasn't a good answer for that. "I... Don't know?"

"What about things like consensual sex between adults? Literally any action that occurs with informed consent between all parties?" he asked. She immediately flushed.

"It's... Indecent!" she protested weakly.

"Why?" he raised an eyebrow. "Who should care? What business is it of anyone else? Gay sex, lesbian sex, straight sex, it's all between whoever engages in the act. Sure, there are risks like STDs and unwanted pregnancy, but again, that's the point of informed consent."

She was flushed, and stammering a bit, but remained unable to refute his points. "...is this really the time to be asking such questions?" she finally asked, looking away as they finished their meal.

"...yeah, you're right, sorry, got carried away... We should continue our day," he said sheepishly, but was internally grinning at the ground work he had laid. They were soon out of the restaurant, and Asia excitedly pointed at an arcade.