“When are you guys going to have kids?”
This seems like a common, fairly innocuous question. Except when the couple you level your query at has been trying to have kids without success. Then your friendly question just becomes another shake of salt on the wound. And there’s no good response for the couple to give in this situation, as they probably don’t want to share details of their fertility problems with you. Or perhaps they haven’t decided if they want kids at all, or one partner does and one doesn’t, in which case you’re inviting further tension into the conversation.
A better question: None — don’t ask. If they want to tell you about their plan for producing progeny, they will.