Alice and Bob have sex because they want children.
Exploited: In the sequel, the new Big Bad kidnaps their children and uses them as bait.
Alice thinks that Children Are a Waste. She tells Bob that she isn't going to have children, and if Bob disagrees, he has to find another girl.
The witch rendered Alice and/or Bob infertile before they killed her.
Discussed: "So, Bob, do you want children?" "I don't know, Alice. Let's wait a while and then decide."
Conversed: "I love this movie's ending." "I know. The happiest endings are those that involve babies."
Implied: At the very end (several months after the witch's defeat), Alice and Bob are a Happily Married couple. Alice's stomach is fatter than before, implying that she could be pregnant. She silently winks at the audience, as if to say "it's exactly what you're thinking". A Spin-Offspring may or may not follow.
Alice and Bob may be great heroes, but they become horrible (possibly abusive) parents.
Alice and Bob still have demanding jobs as well as Chronic Hero Syndrome, so they don't have time to care for their children.
They take parenting classes before their children are born, so they know how to be good parents.
Someone adopts the children, who become Happily Adopted. Of course, Alice and Bob visit them when they have time, and the children understand that their parents love them and want the best for them.