I don't like it either. When I'm playing an otome game, I want to be able to see each LI as a romantic option without having to think about them being romantically involved with someone else.
Even if it's technically part of the story and makes sense for the characters, it can really take me out of the experience. Especially if I have a favorite LI, seeing him have romantic or sexual tension with another character would probably make me want to stick to his route and never look back.
I think I prefer when the romantic possibilities are centered around the heroine, rather than having the LIs develop romances with each other or other characters.