Homewrecker, Bitch, Slut Complete
Becoming a Mistress to a married man is not something to enter into lightly. There are no "right reasons" to induce someone to engage in an affair with you.
But, most often it is the married man who does the inducing. If he loves you, and you love him, he should simply separate and divorce his wife to be with you.
But sometimes that is not the best thing for everyone. If he isn't prepared to do that, then rest assured that he loves his wife, and you are someone dispensable. Do not make the mistake of equating sex with love.
Therefore, don't assume that if he is having sex with you, he loves you and doesn't love her, when the complete opposite can be true. Men can love their wives and sleep with someone else...it simply isn't the same for men and women.
Having an affair with a married man is not a game. It is a commitment with a very high price tag. People can, and usually do, get hurt.