Try to start up a conversation about the relative merits of marrying your siblings and I bet everybody else in the room will suddenly get very uncomfortable. They'll be thinking "Of all possible things we could talk about, why this??"

You're right that taboos also concern actions, not just discussion, but in this case it's probably both.

> They'll be thinking "Of all possible things we could talk about, why this??"

Sure, but the point is that they should just say that, and the person who raised the issue must then explain why it's worth their time. Maybe there's an important point they're trying to make, maybe not.