Marriage is defined in most (if not all) dictionaries as a "legal union between a man and a woman," with an additional definition in some dictionaries that includes a legal same-sex union in some jurisdictions.
As the Supreme Court argues the constitutionality of same-sex marriage, comments by the Court raised the issue of the definition of marriage. If marriage is a legal term, then the definition of marriage is a matter of law. If it is a fundamental right, then the need for legal limits may come into question. Fundamental rights may have legal limits - free speech does not permit speech designed to incite violence or to instill panic; the right to bear arms does not include ownership of all forms of weaponry.
Marriage, then, if a fundamental right, may be legally limited.
Can marriage be limited to couples of the same race?
Can marriage be limited to heterosexual couples?
Can marriage be limited by age?
Can marriage be limited to couples who are not close relatives?
The interracial couple question is moot - the legal right for interracial couples to marry has been affirmed.
Same sex marriage is presently under review by the Supreme Court. A significant majority of the people (61% on a recent poll) feel is should be a fundamental as well as a legal right. Such a right exists in 36 states and the District of Columbia.
Polygamous marriage has not been brought before the Supreme Court. We know it has historical precedent (both Biblical and otherwise) and continues to be practiced in this country (though illegal in all 50 states) among certain Mormon cults. Polygamy is legal in 50 countries.
States have defined a lower marital age limit for a man and a woman - an age which may vary from state to state. South Carolina permits a girl of 14 to marry if she has the consent of both parents. New York, however, requires not only parental, but judicial approval at that age. Is the legal marriage of a South Carolina couple, where the bride is 14 years old, legal in New York without judicial consent?
Marriage is denied to first cousins in 30 states. Six states prohibit marriages between first cousins once removed. Some states, not all, that prohibit marriage will recognize cousin marriages performed in other states. I could find nothing about marriages of siblings. I don't believe marriage between siblings or parent-child is legal in any state - but, nevertheless, such weddings have taken place. Incestuous (non-marital) relationships are legal in New Jersey.
Who can and can't get married will continue to be a question. Will we require a special classification for the legal marriages of the transgender and transexual community? The whole concept of gender identity has become an issue. Should consenting adults be permitted to enter into a legal polygamous marriage? If not, why not? If marriages between closely related individuals are presently limited for reasons of potential deleterious effects in offspring, should such marriages be allowed if direct offspring are impossible, such as the sterility of one of the members in a heterosexual relationship, or if the relatives are of the same sex?
If the definition of a legal marriage is to change, should it be changed broadly, or should each step await its turn in court? If the Court is leaning towards incremental, rather than broad changes in how marriage is defined, these other issues may require future consideration.