I'm wrong or no?

so my bf has two other kids by the same women. No relations at all with her just see his kids and pay child support . He is 21 and I'm 18. & he wants to get marry and start a family. NEXT YEAR!! & I kinda don't want no children with him just for the simple fact . He already have some . So he doesn't know I'm planning on getting the IUD. I honestly not ready for all that. I'm still in my selfish stage??