Sorry if this theory is already out there, but if it is, then I have not found it.
The Returned, excluding The God King, supposedly cannot have children. We know from the annotations that not only is this actually possible, but that Susebron's priests know the secret to it. While reading Warbreaker, it occurred to me that if a Returned was incapable of bearing children, that must mean that this Returned did not want to have children in the first place! Seeing as how the Returned have a subconscious control over their physical bodies. But why would none of the returned want children?
And then it dawned on me. The Hallendren, keep their gods constantly living in luxury and encourage laziness among them. To a people who enjoy such an idle life, wouldn't a body that is physically incapable of bearing children be the ideal from their point of view? It also probably helps that everybody assumes that the Returned cannot bear children. everybody says that gods can’t have children, so the Returned unconsciously sculpt their bodies so that they cannot.
So, according to this theory, everybody in the court of gods is being kept idle, and being told that they are incapable of bearing children, so they unconsciously make themselves incapable of having children. Everybody except one person. One god is being told from a very young age that he must produce an heir, and being told that he is the only god who can bear children. The god king is supposedly the only Returned who is capable of having children, well maybe that is precisely the reason why the god king is the only one who is capable. This also explains why Vo, the first returned was able to have children. There would have been no expectation for him not to, and no system set in place to keep him lazy and not thinking about children.