Q.peach trees and nectarines
Why do they have sap seeping out of fruit?

This is most likely caused by an infestation of fruit moths.
Here are some articles with more information.
http://entnemdept.ifas.ufl.edu/hodges/ProtectUs/presentations/peaches_arthropods.pdf
http://www.clemson.edu/extension/hgic/pests/plant_pests/veg_fruit/hgic2209.html
https://www.gardeningknowhow.com/edible/fruits/peach/common-peach-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/nectarines/nectarine-fruit-oozing.htm