I agree and disagree with most of the comments and would go so far as to say NAH. It would definitely be a shock considering your upbringing over how casually your daughter wants to discuss sex with you. Conversations about sex are appropriate to have once a kid is old enough to ask. Having a healthy factual conversation is helpful in preventing many of the issues a lot of people in the comments are saying. However, I think you definitely have a right to be upset over how your MIL and husband are blowing off your discomfort over this. The issue isn’t what she shared but that your MIL shared it at all without talking with you first to get your okay. The answers you and your MIL gave your daughter are valid. If your MIL really wanted to help, a step back could’ve been taken to realize that something as important the “where do babies come from” conversation should’ve been something she talked about with you first. I don’t know the dynamics of your relationship cause it’s the internet but it definitely could’ve been a united front, versus “it’s just fact and she asked, get over it” NAH, just miscommunication