There isn't a lot of evidence that they benefit society. What studies have been done of them have found that they are difficult to sustain without substantial wealth, the wives often compete so that their children will get the attention of the father, and in some sects, the boys of the family are kicked out of the family because they pose as competition to their fathers for young wives.
In reality the "gay marriage will lead to polygamy" argument is humorous because the opposite is true. Polgyamy has lead to same sex marriage. Polygamy existed before same sex marriage, was outlawed before same sex marriage, and is the first argument brought up when considering same sex marriage. People have effectively used the banning of polygamy as the defining factor for two person marriages. However, the flaw in that definition is that it doesn't justify restricting the genders of the two people getting married to being different. So people have no put the wagon in front of the horse and now argue that what they have used to define marriage, the banning of polygamy, will fall apart if two people of the same sex are allowed to marry. It doesn't make a lot of sense, but meh.
That argument that Polygamy isn't beneficial to society but gay marriage is... Is a farce and everyone using it knows it
I understand, that when some on the right went "Woah, wait, if we use that logic as a basis, won't that just make polygamy justifiable?" The Left realizing this danger has pulled out all stops to both deny this is the case, attack the argument and those using it as stupid, or idiotic... and they refuse to debate the issue as a general rule. See Jallman's behavior, fits it to a T.
The smoke screen, that Polygamous Unions have no benefit, and are too tough to maintain so there for not worth discussing is another common tactic.
Arguments for Gay Marriage.
Gays as Individuals Will Be Better Off:
Studies repeatedly demonstrate that people who marry tend to be better off financially, emotionally, psychologically, and even medically. Marriage is not universally an improvement (women, for example, can actually be worse off in some ways), but it generally is. Because of this, it stands to reason that legalized gay marriage will ultimately prove beneficial for gay individuals. This, in turn, will be better for gay couples, the families of gays, and communities where gays live.
Could you not make the same argument for three people who believe that such a union is best for them?
Gay Couples Will Be Better Off:
Perhaps the most important aspect of marriage is that it establishes a legal and social relationship which makes it easier for people to “be there” for each other — economically, emotionally, and psychologically. Most of the rights and privileges that go with marriage are, in fact, ways to help spouses support each other. Married couples are thus much better off than unmarried couples, giving relationships the ability to grow stronger and deeper.
Would it not benefit a Polygamous relationship if there were no longer a need to hide it? If the people involved were able to utilize the legal protections of Marriage? Would that not remove the stigma attached and let them live in peace?
Families with Gay Members Will Be Better Off:
Because gays can’t marry, it’s very difficult for partners to help each other in difficult situations like medical crises. The burden of support and decision-making typically falls in the laps of other family members when it should fall to one’s chosen life partner. If people know that they can rely upon their relative’s spouse, they can be far less anxious about what will happen to their loved one — not just in the context of a crisis, but in general, too.
Again, would it not better if a family of 3, or 5 be able to make sound legal decisions for their loved ones, instead of running from the law, instead of being unable to make choices,a nd being forced to sit by as others do so for them?
Children of Gay Couples Will Be Better Off:
The Christian Right would deny gays the ability to adopt or raise children, but that’s an impossible goal. Children are already being born to, adopted by, and raised by gay couples in increasing numbers. Children in stable, married households can be better off than those who aren’t because both parents can handle decision-making and parenting without worry. Opponents of divorce often cite the negative effects on children; the same can be said against bans on gay marriages.
Now, currently there are "studies" showing Polygamy is bad for kids. Yes, and there were (and are) studies that show children of gay couples have problems too. As the idea is more accepted, so have the studies shown the problems with the gay family have subsided. Is it not reasonable to conclude the same would occur in a group marriage?
Communities with Gay Couples Will Be Better Off:
Married couples can help and support each other in a variety of ways because laws and regulations are written to help that happen — for example, people are able to take time off to help their hospitalized spouse. Gay couples who cannot marry don’t receive the same help, so much of what gay partners would do for each other must be shouldered by the community at large, unnecessarily draining resources. By solidifying relationships, gay marriage can help stabilize a community overall.
Again, if that "family" of 3 adults and 4 kids in tht big house at the end of the block were no longer freaks, but accepted members of the community, could not their contribution be added to strengthen the community rather then be a point of ridicule? Would it not be better if the ones in the unit were working could provide a group coverage benefit through work in terms of Healthcare instead of relying on the city, or state?
Gay Marriages Will Help Stabilize Society Generally:
Conservatives who usually oppose gay marriage argue, correctly, that stable families are a cornerstone to a stable society. Families are the smallest social unit in society and trends in the family inevitably affect trends in society as a whole — and vice-versa, of course. Allowing gays to marry will help better integrate them and their relationships into society. Ensuring that gay relationships are stable and receive support will benefit the stability of society overall.
Ditto for a group marriage situation.
Gay Marriage Could Benefit Marriage Generally:
Opponents of gay marriage argue that it would undermine the institution of marriage, but it’s hard to see how more marriages would be bad for marriage. If anything harms marriage, it is bad marriages where people don’t take marriage seriously — and that’s already too common with heterosexuals. If gay couples in committed relationships are able to formalize their unions as marriages, that can only serve to improve marriage overall by providing more positive role models.
If 4 people could join in marriage, and be successful, how is that a bad thing?
Arguments for Gay Marriage: Moral and Social Arguments for the Legalization of Gay Marriages