Now that I'm married and achieved the wish of my sim. How can I divorce without fighting my wife ?
In order to break up/divorce in the Sims 2, the couple's relationship needs to be pretty low. There is no happy way to do that. One way is to purposefully be mean and fight with your spouse. Another way is to have an affair and make sure your wife sees it. She'll be pissed and hurt and you can then break up with her.
I suppose not interacting with her for a while would also make your relationship drop and you'd eventually get the break up option, but that's just a guess.
Of course, if you just don't want to be married anymore, you can always just kill her, as Halfwarr suggested...
To trigger the "Break Up" option, the sims in question must have both a Daily and Lifetime relationship of 45 or less.
How the relationship reaches this point can vary. If you don't want to fight with your wife, you'll have to ignore her for a long time - there's a 2 point relationship decay every day, so you'll have to wait somewhere on the order of 2-4 in-game weeks (14 to 28 days) for the break-up option to be available.
But even once you get there, there's no such thing as an "Amicable Breakup". Your sims are bitter, jealous creatures, and will take breaking up poorly, even with low relationship scores (not to mention forming a negative memory).
That being the case, I'm not sure there's much benefit to breaking up without fighting, as you get the same end result in either case.