I think the culture of the world he transmigrated too was just more accepting of same sex marriages (plus the Duke has a younger brother in case he himself doesn't have an heir, I think it may of been a different story if there were no backup heirs since the Duke and MC can't have children (the story hasn't stated how the culture of aristocrats feel about adopting children into the family to be heirs, but I imagine they wouldn't be too fond bc of bloodlines and all))
I can't remember where in the story (I think it was somewhere near the beginning), the MC was thinking about this same issue (he seemed pretty happy with how accepting the culture was in regards to the world he had came from (tbh I wouldn't be surprised if the MC was already either bi or gay (so far I think the only woman he's been "interested" in was the ogFL and it hasn't really been in "oh, she's hot" kinda way and more "I kinda need her, don't I? Welp"))).
Just curious, doesnt the MC shock when him a straight guy, transmigrated to a new world and have a fiance which is a man. Like if you are a manly man you will be agaisnt the idea of having a fiance which is a man （￣へ￣. And also if the novel he got transmigrated into has heroine and male lead, doesnt that mean the novel is straight one or is it an omegaverse so the community accept man to man relation ship? how come the duke has a fiance and its a man, and people doesnt say anything like I think people in that era are pretty prejudice.
anyways I like BL I was just curious(๑•ㅂ•)و✧.