I've never experienced what the other person is saying, with certain plot exceptions, the girls don't seem to care what you are doing in regards to the other girls.
(Though kissing one girl in the same area where another girl you are romantic with will cause a negative hit to the other girl, and possibly set cheating flags).
However, if a girls lesbian stat is high they will lose a number of love points every day until you get their lesbian stat down low enough. (Typically done via advancing their story, though I believe there is a book that does it as well.)