I cringed a little when I saw the tag 'het', but I saw the reason for it. This isn't one of those ongoing, mindless sex doujins, it actually has it's story to it. I see that many of you are having trouble figuring out the reason why Meiling had sex with a guy? My answer would be that because Meiling is a youkai and since she did claimed that she went wild, it would be that her youkai instincts acted up so much that it literally was out of her control to stop her youkai needs. And maybe those needs were for human attention and that it was easier to reel in human males after offering her body, which makes it easy to kill them in private. What guy would not accept a hot girl offering sex? And with that she satisfied her needs sexually and the need to kill. And it makes sense why she kills human males. Human are powerless creatures, easy kills for youkais. And males because they're an easy catch for female youkais.
You guys complicate your minds too much on this. Anything based on instincts is obviously no self control on what you do, so hers were those. She explained already. She didn't do it just because she didn't care or felt like it, it all falls down to no self control. Once she realized it, she started to self retrain her youkai instincts because she never wanted to hurt anymore humans, like Sakuya. When Sakuya touched her, Meiling rejected her because she didn't wanted her instincts to go out of control. BOOM!✴