good question. I don’t know what his autobiography says
From Dreams from My Father, page 70:
“Well, I think it’ll be wonderful for you two to finally get to know each other,” she said from the kitchen. “He was probably a bit tough for a ten-year-old to take, but now that you’re older…”I shrugged. “Who knows?”
She stuck her head out of the kitchen. “I hope you don’t feel resentful towards him.”
“Why would I?”
“I don’t know.” She returned to the living room and we sat there for a while, listening to the sounds of traffic below. The teapot whistled, and I stamped my envelope. Then, without any prompting, my mother began to retell an old story, in a distant voice, as if she were telling it to herself.
“It wasn’t your father’s fault that he left, you know. I divorced him. When the two of us got married, your grandparents weren’t happy with the idea. But they said okay-they probably couldn’t have stopped us anyway, and they eventually came around to the idea that it was the right thing to do. Then Barack’s father-your grandfather Hussein- wrote Gramps this long, nasty letter saying that he didn’t approve of the marriage. He didn’t want the Obama blood sullied by a white woman, he said. Well, you can imagine how Gramps reacted to that. And then there was a problem with your father’s first wife…he had told me they were separated, but it was a village wedding, so there was no legal document that could show a divorce….”
Her chin had begun to tremble, and she bit down on her lip, steadying herself. She said, “Your father wrote back, saying he was going ahead with it. Then you were born, and we agreed that the three of us would return to Kenya after he finished his studies. But your grandfather Hussein was still writing to your father, threatening to have his student visa revoked. By this time Toot had become hysterical-she had read about the Mau-Mau rebellion in Kenya a few years earlier, which the Western press really played up-and she was sure that I would have my head chopped off and you would be taken away.
“Even then, it might have worked out. When your father graduated from UH, he received two scholarship offers. One was to the New School, here in New York. The other one was to Harvard. The New School agreed to pay for everything-room and board, a job on campus, enough to support all three of us. Harvard just agreed to pay tuition. But Barack was such a stubborn bastard, he had to go to Harvard. How can I refuse the best education? he told me. That’s all he could think about, proving that he was the best….”
She sighed, running her hands through her hair. “We were so young, you know. I was younger than you are now. He was only a few years older than that. Later, when he came to visit us in Hawaii that time, he wanted us to come live with him. But I was still married to Lolo then, and his third wife had just left him, and I just didn’t think…”
She stopped and laughed to herself. “Did I ever tell you that he was late for our first date? He asked me to meet him in front of the university library at one. When I got there he hadn’t arrived, but I figured I’d give him a few minutes. It was a nice day, so I laid out on one of the benches, and before I knew it I had fallen asleep. Well, an hour later-an hour!-he shows up with a couple of his friends. I woke up and the three of them were standing over me, and I heard your father saying, serious as can be, ‘You see, gentlemen. I told you that she was a fine girl, and that she would wait for me.’ ”
Of course, that still doesn't say whether they were married at the time of BHO's birth. Only that they got married at some point. Also, I believe divorce paperwork has been found.