Diplomacy - Choosing sides...

[_Tristan_]

I've just noticed that simply choosing sides in a confilct between two of your allies implies a drop in your reputation...

Playing the Long Road mod, I was allied with all 21 factions at the same time having at least very good relations and over with all of them. my reputation was mixed from the start and had never gotten above that level.

A war erupted between two of my allies (English and Turk) and after choosing sides, my reputation dropped to dubious...

Some more of that and I'm now Very untrustworthy without having really broken the least alliance...

What do you make of that ? Do you have an idea on how to raise my reputation anew ?

[EDIT] To be sure, I've always occupied and having fought only rebels until now, never had to choose what to do of the prisoners...

Might it be because I'm the most powerful of the lot ?
I'm disgusted as I wanted to play it cool and obtain the upper hand through diplomacy (an objective that might become a bit difficult if I cannot my alliances...)