It's predictable how closed-minded the arguments on this thread are. Prostitution, money, would you want your daughter...? all that stuff.
Looking at it from a purely moral position, I consider the lying aspect to be more important than who you have sex with. Let's face it, being a wife is often a financial arrangement.
Imagine one were living in a polygamous community. The man would have one, two or more sexual partners. So that in itself is not the issue. (Hi King Solomon and all the others in the bible )
The only real issues are sexual and procreational health, and emotional health.
It's similar with consecutive sexual partnerships. If you've had multiple partners over time, and most people have, then the key issue with regard to actually having sex is sexual health and childbirth, or lack thereof.
So a man living in a monogamous community who takes on another sexual partner without alerting his wife is dishonest. Having said that, is it better or worse than releasing his wife from her financial contract with him? Divorce, leaving a wife for another woman, is accepted in Western society, but not as well accepted in polygamous societies.
However, if a person lies to a current sexual partner (wife, girlfriend, husband, wife) about who else they are having sex with, to me the deception is a problem, not sex.
So to Diana I say, think about how honest this man is being. If he can lie to his wife about meeting you, he can lie to you and everyone else in his life. Whatever you choose to do, stay sexually safe and look after your emotional health.
As Ynot said...
One size morality doesn't fit all.