Those phrases are two different schools of thought.
#1 - Let a man be a man
Sometimes women are so "take over" and controlling that they steam roll others. Some women do this from telling a man what to do, where you will eat, what time to meet, etc.etc. Some women do this and don't even realize (or sometimes they do) they are taking control over everything. Stop bossing and controlling everything. This is about allowing a man to feel useful/needed and less like a big kid. It's hanging up the momma/son relationship...........your man should not be treated like a child there to comply 24/7. Maybe you need some oil in the car and you ask him to do it. When your man is around, you don't have to carry all the groceries into the house by yourself. I'm not saying don't be independent but allow others to lead and help. My best example: If you are in a canoe by yourself you have to row, guide yourself, direct yourself and navagate alone. If you are in a canoe with another person -he can guide, he can help, and give you pointers you can follow if he's familiar with canoe....so you can follow his lead.
#2 - A man is going to be a man
B.S. This is the "old school" way of thinking that means its okay for your man to disrespectfully interact with other women, cheat on you, or treat you like less than you deserve. It's the old "settle for any behavior" which is NOT RIGHT. We all are adults, we all see people who are attractive -however flirting /making inappropriate comments, staring, slipping numbers -unacceptable. Men are visualy stimulated and are more capable then women of separating bodily activities from interest/love HOWEVER that does not give anyone a green light to cheat. Along with this is the excuse of turning a blind eye to cheating with the idea that it's okay as long as I don't see-b.s. This mentality belongs in the trash can. Cheating is not okay. Being disrespected in public with your man flirting or trying to talk to other women around you and make you look stupid for being with him -unacceptable. (This includes the b.s. hookup of trying to give someone a "business card" out of the blue and other stupid moves) You teach people how to treat you and you accept or reject their behavior.