There are numerous reasons for not to get involved with a married man
The primary reason is this: it's just plain wrong. You are cheating a family not a person. Cheating is never right.
You will be always 2nd Best
You will never be his first, you will always be second best. He will always treat his wife and family first. As he has to cover up his cheating, it won't be easy for you to contact him. For any important occasions he will be always with his family not with you.
He is a lier
He will always throw lies to you. Whatever he says, he won't leave his wife. Think logical, Why would he leave his wife and disrupt his comfortable life when he can get the best of both worlds.
Do you like it?
Do you like if your partner date with someone other than you?
Men who cheat can never be trusted.
If you leaves his wife for you, then remember he will leave you when he find another better one.
If he has children, your involvement in his life could be incredibly damaging to them. Don't be a part of messing up children's lives; stay well away. Can you respect a man who puts his own selfish needs before that of the children he brought into the world?
If you do get into a relationship with a married man, you'd better do so with your eyes wide open. There is an almost 100% certainty that it will end badly.