Most Helpful Guy
Its fine if this guy wants to leave his current wife to marry a woman who he can have kids with. However, he is shopping around for his next girlfriend before leaving his current wife, and that is sleazy.
In any case, your original question was how to dump him kindly. You honestly already have. Let him know why you're no longer interested, and stay firm about you decision. If he can't handle being friends (not uncommon for anyone who is human) then stop communication with him.