It's not either-or, it can be both things. I'm an adoptive stepparent. My son is almost twenty.
If your fiance does adopt the kids, it does not erase the truth of who their bio-father is, nor should the children be lied to about who their biological father is. Your fiance becomes their legal father as much as he is already the Daddy who is raising them, and their biological father remains what he already is, the biological father. The adoption might terminate his legal parental rights, but not the reality of the children's conception and birth.
The kids will have questions through the years to understand this, as much as they will have questions about many things. It's part of your job as a parent to help them understand their unique situation...Daddy is Daddy, who makes the decisions and provides for and protects them and loves and raises them as they grow. The man who conceived them is the one who passed on his genetic heritage to them and they can know who he is and pursue a relationship with him when the time is right should they wish to.
The adoption does not necessarily mean that the kids shouldn't have the right to information or even a relationship with their biological when the time is right.
My son has always known who his biological mother is and they've met and are free to contact each other any time and have any relationship they wish to have with each other. At the same time, I'm Mom and always will be. I don't erase her and the adoption didn't erase her. It made me his legal mother and I felt so secure knowing that if anything happened to his father, I would have all the rights of a parent and his life would not be disrupted and we would not be separated. The adoption established the future, but didn't erase the past. Biology and adoption are both permanent realities.
Usually, you have to be married for a minimum time period (often a year) before you can petition to adopt. You will have to notify the biological father and give him a chance to respond. You will need to demonstrate that your marriage is stable and that the adoption is the best decision for the children's future.
Hope this helped.
