Yeah, I think the whole 'be fruitful' thing made more sense back when babies and children died all the time from disease and whatnot (not to be morbid, but it's true) and a woman who had 19 children over her lifetime might have only had a few that survived. This is getting out of hand.
And I'm not sure how often they actually do have sex, if they are the kind that thinks that sex should only be used for procreation (which from knowing their other beliefs may be true) they may go through periods of none while they focus on getting the family settled, then concentrate hard on getting pregnant, and then none again until they are "ready" to have another child.