BTw, for those who seem to be confused about the purpose of Sigheh in society, sigheh is the equivalent of passing out clean needles to IV drug users in some European countries.
When some public health officials support passing out clean needles or setting up clean drug clinics it doesn't mean that either they are dope dealers or that they condone the practice of drug use. It means that they recognize the existance of a certain social reality and try to prevent worsening an already bad situation by pretending that it doesn't exist or forcing it to go under-ground by simply making it illegal.
By the same tocken proponents of Sigheh recognize a certain social reality which has been labeled "the oldest occupation in the world." Men willing to pay for a woman's attention and women willing to give that attention by getting paid have existed independent of any religion and long before Islam or Shiism.
For centuries and centuries and to this day most societies either tried to ignore the situation (by pretending it doesn't exist) or tried to solve it by force and passing laws that would make the act illegal. Thus for centuries and centuries the prostitutes have been the lowest class of society simply without any legal rights while the men who paid them or their pimps could get away with abusing them, torturing them, beating them, not paying them, and simply getting away with it because the society had chosen to act like the problem doesn't exist.
In this situation comes the idea of Sigheh which says wait a minute. If men are contributing to this social dilema why not make them take responsibility for their action by making the whole thing legal. By making the act legal, the man can no longer take advantage of the woman and not take any responsibility for his action. The woman will feel safer and less chance of being abused by other men. If there ever is a baby borne in this situation there will always be documentation as who the parents are. And last but not least, a woman can no longer accuse a man of raping her after she has signed a paper in front of a witness and agreed to have sexual relations with that man.
Again, just as the proponents of a clean-needle program doesn't necessarily support drug use, the proponents of Sigheh don't necessarily support the idea of extramartial affairs but rather, they both recognize the reality of existance of a social problem that needs to be recognized and dealt with instead of ignored or simply dismissed.