Your own survival is crucial, and if you do happen to fall in love with a married man, there are several hard truths you need to know. The needs of the many (namely, his family) will always outweigh your needs.
His family will always come first, and that includes his wife. The beginning of an affair is romantic and naughty at the same time.
In fact there usually isn't after the affair is over. Even though he has a deep feeling of love for you, he is able to process it in an unemotional way.
Being involved in an affair with someone else's husband is an almost surefire trip from ecstatic highs at the beginning to a depressing abyss at the end.
Understand the basics of exactly what you are getting into, and what your status is.
He knows that holding on to emotions that can only cause problems for his family is something he cannot and will not do. To safeguard yourself from too much emotional pain, you need to understand that he can only be a small part of your life and will never be more than that no matter how many promises are made.
You need to have a life that works and that is full enough to withstand the pain of the eventual breakup. A solid circle of friends and a social life separate from your hidden life with him is a necessity.
And don't ever kid yourself on this important point: He is still having sex with his wife, no matter what you may want to believe. Legally, financially and emotionally, you have no claim.