Regarding the all white Bridgertons and Featheringtons, I am guessing they are trying to keep within the bounds of making the families believable in terms of genetics/biology -- for instance, consider the all black Bassett family -- but that doesn't mean there aren't mixed race members, like Marina and, if Daphne should get pregnant, Daphne and Simon's children. At least, that's what I'm thinking.
Regarding Simon hanging around to see if Daphne is pregnant, he wouldn't want to abandon her if she indeed were pregnant. He doesn't want to be a father -- allegedly -- but he doesn't want to leave her and his child without his support and protection if she were. Otherwise, he proposes that they part so that it doesn't happen again.