I don't know if i necessarily agree with Mr. Ralph Waldo Emerson. I know of a few life situations that have made people make drastic changes to their character. (i.e. birth of a child, death of a loved one, natural disaster, etc..)
There is a saying that says "the only thing constant in this world is change". Basically change is inevitable. I guess it all depends on what type of change takes place.
So knowing that change is inevitable that doesn't necessary mean that the person will make a change to his/her character.
But to answer the question posed "Once a cheat always a cheat?" In my personal opinion, I don't believe this to be true. Yes, some people will always play the field and will never settle down with just one mate. They may have the mentality that "God didn't make man to be with just one mate, it's just not natural" Or they just don't care they may feel as though they can't help it.
But then there are those who may mature with age and feel as though they are ready to just be with one woman/man. They had fun as a youngster and now it's time out for the silly games.
There is a scripture in the bible that says "when i was a child i spake as a child, i understood as a child but now that I am a man, i have put away those childish things"
This scripture speaks volumes.
So yes, I believe that there are some people that do change and then there are those who will never put down that "playa" card.
We all know that men mature later in life than women, It's scientific fact. So whereas a woman may be willing to commit herself to one person at 22 that man may not be willing to commit until 32.