"Men will always be men", this phrase simply refers to the certain behavior that is common among men. It talks about the mindset most meN carry. This kind of behaviour is to be expected sometimes.
For instance, men cannot resist looking at a beautiful woman no matter how beautiful their wives are. This does not mean they intend to cheat but this behavior is already in their mindset. Another simple example is the love men have for football.
Men are often found to have a one-track mind. Often women have seen that men cannot resist looking at another woman, no matter how beautiful their own wife. At the same time, they are also immensely possessive about their own wife is. This is why it is said that men will be men.
Certain brands have also portrayed this message in a whole series of ads. It signifies that no matter what you try, you cannot change the primary wiring of the male brain. They will think along the primitive lines they are made to think, even if they have the most beautiful woman standing beside them.