It just creeped me out, is all.I'm sure that this is just me taking a joke too seriously again, but I don't think that's really going to be an issue for anyone in-universe. Yeah, Rin met Sesshomaru when she was twelve and had children with him, but it's really hard to take it as a serious kind of issue when she married a demon that is hundreds of years old. I can understand still not liking it and I'm not saying that you have to be okay with Rin and Sesshomaru getting together, but since her disappearance is probably going to become more important, she is probably going to be mentioned and/or shown as their mother more over time.
I've read a bit of Ranma 1/2.I think not introducing more romance into the series right off the bat was a good thing. Admittedly, I'm not too familiar with most of Rumiko's other works, but I know that romance is often one of the main driving force of her series. Since the romance between Inuyasha and Kagome was one of the main defining features of the original series, I think it was a better choice to have more of a monster of the week kind of format with the girls instead. I'm not against romance by any means, but I think it helped to make Yashahime more distinct from Inuyasha. If all three leads had romantic interests, I think it would have come off as trying to mimic the original series instead of being its own thing, if not just trying to juggle too many characters at once.
It's basically a love polyhedron with a gender-swapping angle.
Yashahime is definitely one of those series that doesn't need to be bogged down with romantic interests. Maybe as a plot for a one-off story, but otherwise, sisters before misters as far as I'm concerned.