There are so many stories now a days that also turn straight men gay. It’s not homophobic intentions but rather to just add to the plot. You see Lucian change from toxic to healthy and that her whole isekai changed the story so don’t read into it so much
Bisexual people also exist, isn’t not mentioned at any point that he’s strictly homosexual. He fell in love with Rachel for the same reason he would have fallen in love with Noah. I think that reason was always more important than their gender, although I do admit it feels like a very unnecessary distinction to make both couples (Duke x Liam and Lucian x Noah) in the story gay only to change it later.
It feels icky they turned a gay man straight and had him fall in love with his adoptive sister like come on :/