Here's another perspective.
Forget the rules; the "shoulds" and "shouldn'ts"; and all the little decisions that led to where you are today; all the things that may or may not happen in the future. Focus on what is happening now.
You love someone - which is a good thing. He loves you - to what degree I don't know, but after five years you surely do. He also loves his wife and kids. I am assuming that the wife doesn't know about the relationship with you; if she does that makes the situation a little different, but not much.
There are two big problems sitting here. (Lots of smaller ones as well, but let's focus on the big ones.)
The first issue is that he can only give you a small slice of his life. That's the deal you're making in this relationship. It sounds like that deal is no longer working for you - you want more. But here's the reality: you can't have more. If you want him you have to take him on his terms, (and he has to be willing to take yours).
The second issue is that if the wife doesn't know, then you are sitting on a ticking bomb. Because this nice little space that you two have carved out for yourselves doesn't really exist - it's a mirage. As yussamahal said, it's just a matter of time before the truth comes out. And when it does it's going to be painful and destructive for everyone concerned, including some very innocent bystanders.
What I see in your short message is that you love him, but the relationship is no longer working for you because you want more. You really only have two choices: leave now, and be miserable for a while with the loss of that hope of love, or wait for it to blow up and be even more miserable PLUS add pain and suffering to others involved.
But the bottom line is that you can't get what you want from this man. It doesn't matter how much you love him (and I'm sure it's a lot) or how beautiful he is. It sounds like you are young, so let me just add that you can love someone truly and deeply, but unlike in the movies, true love does not always lead to "happily ever after". That doesn't make the love any less real. It just means that we have to respect ourselves and our needs, aspirations and limitations enough to walk away.
If your desire is to love someone completely and have them love you back the same way, then get out of this dead end so you can heal and be available for someone who wants the same thing.
Of course, this is all just my opinion given from a great distance.