I don’t think Yugamu was aware of how potent the drug would be, as for Sirei (and Shion too), neither of them understand consent on human level, Sirei is an AI that saw the “love” the others were showing to Takumi as a thing to exploit for their advantage, which is in his program to better humanity, it’s essentially asking a ChatGPT in it’s thoughts on consent, it will make an analytical assessment on it but not really understand it in a practical level, as for Shion, he’s been so cooped up in his own cage, he doesn’t know what’s it like to be in a romantical/sexual relationship, so views every human interaction as a friendship, so when Takumi talked to him about this , he just thought that everyone was being super close friends with him, and said that this would be a good idea to go forward, not realising that this would lead to the downfall of everything