This is the process for determining whether some law/restriction in law violates equal protection or not.
First, the person/group who believes their rights are being violated challenges the law in court. Which court system it starts in depends on which level, federal or state, the law would being violating the guaranteed rights on. So if the right is being challenged under a violation of equal protection guaranteed by the 14th Amendment, it would go to federal court.
In the case of equal protection, there are two parts. First, the court must determine if the people are actually being treated unequally under the law when compared to another, similarly situated group. Then, the state has to show why it is treating those groups "unequally", because they technically are legally allowed to do this as long as they are able to articulate why they are treating the groups differently, what state interest is being met in this unequal treatment and how exactly the unequal treatment actually meets the state interest being claimed to be furthered by it.
This is why every case is different. This is why the Loving decision did not automatically legalize same sex marriage or strike down the law saying that those behind on their child support could not get married or that prisoners couldn't get married without warden approval. These laws had to be individually challenged and proven to the court as to why the state's argument did not hold water for upholding the interest they claimed it did.
So then we go to each individual law that may be challenged pertaining to marriage and why they are all different. Allowing same sex couples to marry is different than allowing interracial couples to marry, not by much, but it is different. So it had to be challenged. Just as the child support/marriage law was (Zablocki v Redhail). Just as the prisoners needing warden permission was (Turner v Safley). I absolutely believe that polygamy laws will be challenged again, as they have been in the past. But they still would need to be challenged. Incest laws will likely be challenged, although it is likely they would only be challenged for specific relations, not to totally abolish these restrictions.
Once challenged, the state can make its arguments for why the restrictions are within the interest of the state, furthering a state interest. These are where the biggest differences will lie. Many people are treated unequally by laws but they don't make it far in the courts if to them at all because lawyers and judges know that the state's argument for why the unequal treatment is needed will be good enough.
Whether right or wrong in the past, we have reached the point where same sex relationships are legal and the state's arguments for keeping them from entering into marriage cannot be supported as actually furthering a state interest. With divorce, alternative methods of procreation, childless couples at a level in the US of 25%, DNA tests to keep children from being "bastards", remarriage, birth control, equal treatment of the sexes, and many other things that have changed in the last 40 years, it can really no longer be supported that marriage is for children, whether it was about children in the past or not. The religious/moral argument doesn't work. And even the Congressional Budget Committee has done a study that determined that same sex couples being allowed to marry would likely benefit the government budget, not harm it. You cannot prove "harm to society" and same sex couples being together does not harm those individuals or anyone else.
If we look at the other "groups" that may be treated unequally as far as marriage goes, the arguments for why restricting them would be different than those concerning same sex restrictions. Multiple spouses would cause major legal issues. Whether that should be enough to restrict them or not, would need to be decided elsewhere. Personally it is enough for me and it is a reasonable argument that would not apply to the current number limit, one spouse per person. Whether an immigrant spouse is male or female, black, white, red, or blue, it would still be just one person for the government to look into for each person trying to marry them (one example of legal/financial hardship on government for multiple spouses). It becomes a hardship for society though to check to ensure these couples are not committing fraud when we are talking 4, 10, 40 spouses, and we have already determined that it is important to society that these marriages should be checked out to ensure the people are not getting paid to get their "spouse" citizenship. If such a reasonable argument exists on a personal level for same sex marriage for individual people, fine, oppose same sex marriage for that grounds, but it shouldn't be because of why others might or might not support other types of marriage. But don't expect the Courts to agree with you. If they don't agree with me, I'm willing to accept that. But something like polygamy shouldn't be legal just because same sex marriage gets made legal. Incest too has its own arguments against it. Now, I don't think those arguments are strong enough when it comes to first cousins not being allowed to marry, but are when it comes to closer relations not being allowed to marry.
