Is Mutah allowed for the married persons?

One of the rational arguments that is put forward in favour of *Mutah* / temporary-marriage is that it helps to keep one away from falling into the sin of *Zina*, when he is not able to marry permanently. Now, my question is: - Is being a bachelor/divorced, a condition for legality of Mutah in Shiite Fiqh? Is it allowed for a married person to do Mutah in his normal life? Or is it allowed only under some situations, like when a married person's spouse is suffering from a long time ailment or he/she is away from home for a long time? Another related question would be: - Is it a man who goes into seeking a relationship in Mutah or the act of seeking is valid from both directions?