If two people agree to go indoors they walk at their own speed. Now if you have one of those speedy girls, she will make it into the hut and out long before the guy gets there. The thing is that at the time if when you put them together and they are going to have a baby, the number of villagers changes at the same time that they agree to go indoors. So the stats have already been registered. Now if they are walking to the hut and the number hasn't changed they aren't going to come out of that hut with a baby. So if you are trying to get children and the number doesn't change you don't have to wait for them to go in and out the hut. Just put them together 3 or 4 more times and one of those times it will announce that - she nursing, and you say What? and a baby will pop in her arms right on the spot because after she is registered as a mom, you can't make her do anything else. So she won't even try to go into the hut if you mover her after the number of villagers changes. I have taken this way too serious... I know.

For the 2 guys and one girl. When one of the group got split up from a pair due to speed then the other guy might be going in there later at the same time that the other couple does. It happens more often when you have a young female and an old man. It doesn't seem like they can ever get the couple inside at the same time.