Just to clarify: Soleil is not straight, let alone bi. She constantly flirts with women (Ex: In the ending of the chapter where she's introduced, she's seen flirting with a villager. When Kamui mentions the magic drug, Soleil reminds him that she is into girls and not guys and, if I recall, there's a support conversation in the original where she and another character swap bodies that results in her wanting to check out said character's body). She has a unique ability called "Girl Lover". Short version: She likes women. Her not being able to have a same-sex relationship in the game is baffling, let alone the other support conversations that take a very drastic turn from their A to S support levels.
That said, removing the magical drug and giving her a same-sex option would be one of the better solutions.
1. He didn't ask if she wanted his help with what he perceived as a problem that needed to be fixed. She was was drugged without her consent. That's not okay, especially since it was the drug that would eventually lead to Soleil falling for MaCorrin. It doesn't matter what his intent was: Drugging someone without their consent is not okay under any circumstances.
2. Corrin may have known the effects of the drug, but he didn't know if they could have had any other side-effects on her, be it physical or mental. For all he knows, this drug could have killed her due to a severe allergic reaction, like sneaking in peanut butter into the sandwich of someone who is allergic to consuming peanuts in any form.