Even though your question appears to be simple, as per the provisions of Hindu Adoption and Maintenance Act, it puts some problems. Any how, My response to your question is that (1) if you are a hindu unmarried woman (2) the difference of age between the adopting son and you will be more than 21 years (3) If both the natural father (your would be husband) and mother give their consent for adoption, then you can execute adoption deed in the presence of witnesses and get the deed registered. Then the boy will be your son, but the cliche is that his natural father becomes his adopted father. After adoption of the boy, you can marry the natural father of the boy. Let us look it in other way - if first you marry the boy's natural father, then you are not entitled to adopt a son at your free will. Only your husband can only adopt the boy with your consent. And in your case, it becomes that the natural father is adopting his own son- which is absurd and non-permissible. So, my opinion is that first you adopt the boy and get the adoption deed registered and later on marry the boy's natural father, to solve your problem. You are also advised to contact any local advocate well versed in adoption laws, for second opinion.