Sforno on Genesis 17:13/ספורנו על בראשית י״ז:י״ג

1בבשרכם; the word בשר is a euphemism for the reproductive organ of the male, as for instance in Ezekiel 16,26 גדלי בשר, "with outsized male member." Or, Leviticus 5, 2-3 זב מבשר, "an uncontrolled seminal emission from his male organ." There are more examples of the word בשר having that same meaning. Seeing that the organ on which the covenant with G'd is marked is the organ instrumental in ensuring a man's continuity beyond death through the genes of his offspring, it is appropriate that the covenant which represents the eternal intimate and reciprocal relationship of G'd and the Jewish people should be symbolized in this manner.