A Line I Won't Cross? in Life After 60

Back in my wild high school days, I used to say I had morals but no limits. There were certain lines I would not cross. It was like those unwritten laws between friends or as I called them the 10 Commandments of Friendship. One of which was you do not date your best friends ex. I am guessing the adult equivalent would be you don’t date your best friends ex-spouse.
Another one of those commandments was you do not make a play for someone who is already in a relationship. (of course, I saw this one broken so many times I lost count.) Moral lines. For the most part, you respected them or you ended up on the wrong end of a fist. That sacred trust between friends was not to be taken lightly.
Today I find myself at the edge of one of those lines. But at my age, you come to a place where you care less about the lines and more about your own happiness. Fortunately for me, it is a friend from high school. Misfortunately she is married. This is a double, double-edged sword. The back story is long and to many wouldn’t make a difference.
For over a year now, I have asked myself, is there a line I will not cross? What is that line? Can I really have morals but no limits? Yes, I want to be able to have my cake and eat it too. The thing about us humans is that we will justify anything we want. Some would say I have crossed the line already. Others would disagree. Maybe I need someone to throw me a life line.