I think he means there's a difference between births arising from couples in love, as opposed to births arising either by accident or from couples bound by othe

I think he means there's a difference between births arising from couples in love, as opposed to births arising either by accident or from couples bound by other arrangments forced out of motives of survival, hence the qualifier being a "lover's birth rate." As for quantifying which births are "lover's" births, and which are not, well... good luck with that.