Lila, my sweet . . . If he loved you he would not just basically say to you "this is the way I am get used to it". You entered into a monogamous marriage with him and he feels he has changed. If you are not wanting to allow another into your marriage via liason or partner than to you it is still a monogamous marriage and if he "does what is in his nature" he is commiting adultry and intentionally hurting you - because HE HAS BEEN TOLD by you that you do not approve and want nothing to do with any non-mono relationship. If he has changed and can't control himself than he should do what is right and dissolve the mono-marriage because if he stays with you he will hurt you. This would be a sign of love: amicable parting where you both can find love again and your child doesn't suffer.
He is being selfish by expecting YOU to cope with his indiscretions and weakness. Don't let him hold your marriage over you as leverage to stay together. By saying he will still pursue "his journey", he has basically told you he is willing to abandon your feelings along it . . . THAT is not love!!!
Love & Strength to you Dear! I know this is hard for you. Just remember, that anyone who loves you should take into account your feelings and that thier love for you should not be conditionary. <3
"Love is the condition in which the happiness of another person is essential to your own." ~Robert Heinlein