If your wife feels she may get emotionally attached even though she is married to you. I don't think this is a very good idea.
Your wife has already basically stated she is vunerable towards getting emotionally attached. How would you feel if she did sleep with another man, then months later she tells you she is in love with him?
As she is interested in doing it, she has to realise it is solely about sex and that's all. Her emotional attachment should only be for you as her husband.
I'd suggest a lot more thinking about this before making any decisions.