Dr. Sam Thomas is something of an idealist. One of the few African American doctors in 1930s Harlem, he has a deep, genuine commitment to the reproductive rights of women.
That being the case, he responds enthusiastically to Delia's plans to open the first family planning clinic in Harlem. Delia herself is something of an idealist. A follower of Margaret Sanger, the famous birth control activist and nurse, she's fully committed to the cause of female equality. The irony here is that Sanger was actually an advocate for eugenics, the belief that the human race can be improved by selective breeding.
Though there's no denying Sam's commitment to the project concerning the clinic, his motivations are somewhat mixed. There's no doubt that he's attracted to Delia, an attraction that only grows as the play develops. On a personal level, Sam likes Delia, but, as is so often the case, the personal melds with the political.
One gets the impression that Sam has been roused into action by his feelings for Delia. After all, the family planning clinic is her idea, not his, even though he wholeheartedly supports it. Perhaps it's the case that, by throwing his support behind the plan, Sam's trying to impress Delia.
As is often the case in such situations, it seems fair to argue that there are mixed motivations at work here.