There could be a role playing out here too. Sure the tops could announce the coming breeding, in case the bottom doesn't want it or changes his mind at the last minute (bareback but not breeding? Why?)
But I also think it's acceptable, even a turn on, for the tops to just unleash their seed when they feel the time is right. Nobody is going to be surprised or shocked at this outcome. You don't really need his permission. The time to negotiate and communicate those details is NOT 2 seconds before he gets knocked up. If you're the bottom and that's your plan to avoid cum in your ass, you've already missed the boat, bigly.