
I feel like it was a way to get him to stay because of course the MC is stubborn enough to not believe in “past lives” and allat…. I mean I wouldn’t… So he had to make all those things happen to lead to him getting his memories back. He had to be in the environment and also with him(snake guy)… About the ‘slut’ part, he was probably just pissed he was having sex with women all the time in the present… He did that to stop him from having sex with women too… He was probably rage baiting him? In my opinion, the MC wouldn’t question it either because they both loved each other(in the past life) and within 1 month of being with the snake, he caught feelings.. And he promised to remember him till he returns and do all he could to never let him go.. Honestly, I would do all I can to get my lover back. He did that because the MC wouldn’t believe him if he told him he was his past lover… He had to lure him in(snakes being manipulative and all) using dreams, being there to save and help him during his time of need not to forget other people couldn’t see him except that shaman’s daughter…. MC knew he wasn’t normal but still fell for him… So that basically sums it all up…

I kinda agree with you, at least, if you are a “slut” it’s because you had the choice to enjoy sex, but if you were told you were treated as a mere toy, raped and that your rapists killed your family and lied about that, it would be a total trauma, from which I wouldn’t be able to overcome easily ╥﹏╥
So ML is a fuking snake told the MC that he was a slut in his previous life, that he was obsessed with sex. Then in side stories we are shown that MC was raped and taken advantage of by the villagers, he was not a 'slut'. Also I'm 100% sure the MC's nighmares was all ML fault. He was visiting him in dreams and raping him. What the actual fuck this fuking snake!
https://www.mangago.zone/read-manga/that_sexapparition_mgs/uu/br_chapter-284112/pg-36/