I feel that Youta is treading on the fine line between relationship ethics and existential safety with full knowledge when he says that he'll mate with other girls. He'd choose to stay with Lumi (and fck each other over and over like a happy loving couple, not like what he told her, but it's close)
Lumi is rightfully displeased, considering the warm fluffy sweetness of being an OTP, but chooses to hide the pain... maybe because she sees that such acts could save those she cares about. Yep, even at the visceral expense of her beloved sleeping with another girl (...goddammit this is NTR, I can't believe I didn't notice)
Man, stuff gets icky the wrong way when you actually care about them getting along at a fundamental level. This isn't just about fcking anime girls, staying incognito, and lifting a worldwide curse.