PJ -- do we have a DUFU candidate here?
While its possible, its really not an unheard arguement.
Check out Peter Singer, the ethicist at Princeton, he's given lectures and written articles, that actually follow through on the logic and reasoning of some of these court cases, and these questions.
If one walks through the halls of acadamia, and brings this question up, the variety of answers, in various ways, and the debate they engage in, will either frighten you, disturb you, or you'll be one of those guys who starting looking at the family weird.
For the record, one of my former college profs saw nothing wrong with marriage of any kind as long they were both legal adults (I'm gonna skip how she wanted to even determine what at adult is) regardless of relationship, and she used abortion, and various kinds of biosurgery, etc to back herself up.
I puked after that class.