In It, by Stephen King, all the Losers parted after 1958, and grew up to have successful lives. However, they found out to all be unable to conceive children.
Why? Was that It's doing, or the Other's doing? What was the purpose of that?
They didn’t fully grow up. The force that brought them together in the summer of 1958 wanted them to kill It. They tried but the task was left unfinished.
Things that tether them to their childhood are evident in their adult life: Bev now suffers domestic abuse from her husband rather than her father; Eddie has married a woman very similar to his mother; Richie is still the joker, Bill writes about his childhood (whether he remembers it or not). Ben has shed his childhood defining weight but his other interest in structures (the shape of the Derry library, the structure of the smoke hole, the dam) has been transformed into a career in architecture. Stan and his wife have been trying to start a family. Ben is single (waiting for Bev), as are Richie and Mike.
After It has been properly dispatched, Ben and Bev marry and are expecting a child. In the miniseries, Mike narrates this with "another curse broken". It’s a confirmation that It is really dead.
The lack of Losers Club babies also makes the return to Derry easier for the main characters. Their strongest friendships are the ones forged in Derry and their ties to the rest of the world are not as strong. If any of them had their own children, they would be less likely to return to Derry on the basis of a childhood promise.