Rashi on Numbers
ושכב איש AND A MAN LIETH [WITH HER] — a man, thus excluding a minor and a being that does not come under the term איש (i. e., an animal) (Sifrei Bamidbar 7:2; Sotah 24a).
Sforno on Numbers
ושכב איש אותה, and another man has lain with her It is characteristic of the evil urge to cause the sinner to proceed from a relatively mild sin to ever more serious sins.
Rashbam on Numbers
ועד אין בה, to testify that she had been guilty of adultery,