Booth Girls, a Love Story by Joan Uda

Are any of us ever quite what we seem? Kenny Dalton isn't, though Mary Louise Meyer has lived with him in graduate school for three years and he has repeatedly asked her to marry him. Kenny doesn't exactly lie to Mary about the other women he sees, but he does attempt to cover his tracks, and he hides other significant facts about himself.
Mary refuses to see Kenny's infidelities, preferring self-willed blindness to losing him, which she's sure will happen if she confronts him. Mary is a small-town Iowa girl, inexperienced when she slips into Kenny's bed the first time and soon becomes pregnant.

Kenny is studying astrophysics with the famous James Van Allen, discoverer of the Van Allen radiation belts. Kenny's special interest is rocket propulsion. America's first space walk is 15 years in the future, but Kenny knows it's only a matter of time, and that he can be part of the American space program.
With Kenny's brilliant future unrolling in front of both of them, Mary knows Kenny will overwhelm her for the rest of her life if she doesn't walk away. She choses to have her baby at the Booth Memorial Hospital in Des Moines, Iowa, and give it up for adoption.

In 1954, no test exists that will prove Kenny's paternity. Thus he has no legal rights with regard to the baby or Mary. But he's not done with either of them: Kenny doesn't give up on what he believes is rightfully his.

Brief excerpt:
"You never really get it, do you, Mary?" Kenny said. "I would never marry you just because you're pregnant. Five years ago, I might have. I was still my mother's wide-eyed boy, fresh off the farm. I'm different now."
In my mind sweet-sour images formed of what my life would be like if I married Kenny. I saw myself walking down a street clutching a toddler's hand, pushing a baby carriage with a tiny infant in front and a slightly older baby in back, all Kenny look-alikes: eyes, hair, physique, and maybe even personality. And Kenny fondling me to begin another one, murmuring, "Nature abhors a vacuum," while the life I'd planned for myself drained away like so much rain after a gully-washer.