Just don't stop bringing them together. Although they will argue the first few times, or sometimes the first lot of times, eventually they'll kiss and go off to try for a baby. If that attempt is unsuccessful wait about fifteen minutes then try again. Then again if necessary ... I've used Baby Boost a couple of times only, but in general they've not needed it.

Out of my seven generations so far only one produced five kids; the others have all had six. I've had one set of twins in that lot.