This is a widely debated issue in the social and legal debate regarding sexuality. The more legalized an issue, such as in the case of heterosexual marriage the more accepted it is, and this is the underlying fear behind the legal challenges that have been waged both for and against homosexual unions in the present day. Marriage is a legal state, and has been for most of written history, it is therefore difficult for many to allow the legal definition of homosexuality to become one that is an accepted alternative to the broader world, through legislation. Legally homosexuality, is as socially defined by the act of choosing to establish a sexual partnership with a person of the same gender as one's self rather than with the opposite gender. Historically, the legal aspects of homosexuality must be addressed in order to come to an even partial understanding of the similarities and differences between the social and legal definitions of homosexuality. The similarities, between the two are striking, as the legal definition is once again engrained in the acts of sexuality performed that stress the deviance, see sodomy laws, most of which are rarely enforced unless a crime such as rape has been committed, yet are still there, potentially even to be enforced upon heterosexual couples in consensual relations, and for many an even more frightening concept to be...

Even when they believe the behavior deviant, and yet are willing to respond vehemently to legal issues that attempt to allow them civil rights as a minority or otherwise. Though most people in the U.S. are agreeable to civil unions (a legal distinction) possibly as a result of personal social acceptance of homosexuals they are often quick to point out the need to make such unions un-equal to marriage between a man and a woman. The reason for this is as convoluted as the definitions themselves.
References

Baird, R.M. & Baird, M.K. (Eds.). (1995). Debating the Issues. Amherst, NY: Prometheus Books.

Barnett, L.D. (1993). Legal Construct, Social Concept: A Macrosociological Perspective on Law. New York: Aldine De Gruyter.

Johnson, P. (1995, January). God and the Americans. Commentary, 99, 25.

The Marriage Amendment. (2003, October). First Things: A Monthly Journal of Religion and Public Life 14.

Zingo, M.T., & Early, K.E. (1994). Nameless Persons: Legal Discrimination against Non-Marital Children in the United States. Westport, CT: Praeger Publishers.