Yang Kai has always been kind of a sexually active guy, he's always showing physical interest, kissing and touching other girls and Su Yan is perfectly okay with it because she knows she's the main wife. In fact she encouraged him to hurry and claim Xia Ning because she liked her as her sister. This manhua is literally harem so I don't get why y'all are confused about him sleeping with some random woman. I think he also rejected the virgins because he knows he's not going to be involved in anything serious with any of them and doesn't want to be the one to deflower them. That one time he deflowered that Bold Independent Union girl was because that was the only way he could get that charm demon to let her guard down and kill her.