Well, I wouldn't mind being the male subject in such a study; but I
surely wouldn't want to be the female(s).  I suspect that the number
of pregnancies begun (during nursing) would be high.  :-)

BTW, when mothers are nursing babies, there is a decrease in genital
lubrication, much like what happens in menopause.  This is due to
suppression of estrogen by the feedback action of Prolactin (which
stimulates milk production).  Women who nurse their babies for a long
time (more than 6 months) may develop the same atrophic changes in the
vagina that elderly women get, and this may interfere with sexual
function. It usually reverses when nursing is stopped.

When there is no milk production (such as when a male stimulates his
female partner's breast), lubrication results from a neural reflex in
the same way that a male gets an erection when he is stimulated.

If you want to study it, Danilo, be my guest.  Just be prepared to
support a large number of children.

Best,

Bob