Holy cow..
I usually just breed like there was no tomorrow. In my most recent game it saved my ass. Started with a custom ruler, and up having: 4 legitimate sons, 2 legitimised bastards, 3 daughters and 2 acknowlegded bastard sons
5 years after my ruler died (in his early 50's, oldest son was 30), I was down to the 2 legitimised bastards, 1 daughter and the 2 bastards. The legitimate sons died in very rapid succession of each other, but luckily I had legitimized 2 of the bastards, otherwise it would have been a very short game xD
But I must admit, that when having the lustful trait, I find it kinda reasonable that you can't say no. Would be kinda gamey if you could just get a 20 % bonus to your fertility without any real drawbacks.