A part of it is saying that this is an excellent and natural form of birth control. The longer he (the big boy) sucks; the harder it is for (attractive) mom to get pregnant again. You must remember that this business of having one baby after another is not old but new to our world, the human world. Women who lived in the early, mobile, and communal societies did not have the time or energy to pump babies out at a yearly (or biyearly) rate. Breastfeeding helped place/space a good five years between each dangerous pregnancy.
And, yes, the image is, with good reason, erotic. Once again, Sarah Hrdy:
The baby sucking on the receiving end of this let-down reflex brings with it pleasurable sensations, bordering on and blending into the erotic. Whether or not the earth moves, these are powerfully conditioning sensations. To classify maternal sensations as "sexual," and therefore in puritanical minds to condemn them, is to privilege sexuality in a very non-puritanical way, implying that sexual sensations are more important than the equally powerful sensations that reward women for caring for babies.
This statement, like so many in Hrdy's papers and books, is simply profound. Please read it again, if you missed its substance. You need to impress its meaning on your mind. These are the kinds of insights that only talented sociobiologists can provide, and there aren't too many of those in the world.