The doctrine regarding paternity of children born during a marriage to which you refer is legally a presumption, not an admission. These are two different concepts under the law. Marrying a pregnant woman is not technically an admission of paternity.
I happen to agree with you that the most plausible explanation is that he likely did marry her (or agree to pretend to be married) for the reason that at that time he was under the impression he was the father.
I think he came to suspect otherwise and thereafter did his best to ignore her and the child.
Stanley Ann spent her entire adult life, beginning in Seattle in 1961, proclaiming the BHO sr was the father. She talked about how she couldn’t follow him to Harvard because, she alleged, there wasn’t enought financial aid. Obama at Harvard, according to the INS documents, agreed that he had a baby through Stanley Ann.
When did Obama Sr get wise? Why do BHO jrs relatives in Kenya to this day claim relationship through BHO sr? Why was Stanley Ann accepted by his family when she visited at a later date? (I don’t have the exact date of her visit, but it is documented to have occurred)