The relations in this game seem to be a bit odd at times....I am in a rush so I will just give to examples quickly, then more later.
1. After one marriage you cannot get any more good oppinion from a leader (or any father for that matter)....I figure the more marriages the better the relation no?
2. When you are involved in a plot with someone (active or inactive) you should also get at least a little bit of a relation bonus with those involved....and perhaps more when it is actually activated. ESPECIALLY if you accept the plot from another....I mean in that case you are risking your own honor (you literally loose it) and much more...so why won´t the person that asked you to join at least think much better of you after that.
If you'd get a relation bonus, then the plot wouldn't be secret.