Why do guys feel threatened by strong girls?

I'm a guy myself and when i was working out in the gym, there was a girl there that was pretty muscular and was lifting heavy weights. I could hear comments like "i think i can still beat her up" and "i could kick her a.s.s". Those thoughts didnt cross my mind once. i was just thinking how awesome it was for a girl to actually be confident enough to break societal norms and do what she wanted. If she wants to be strong, she has every right to be. why do guys need to justify their manhood by saying they can beat up this girl who clearly was just minding her own business?