First of all, there is no such thing as a legal right. Rights are above the law, and the law may not infringe on those rights.
Secondly, I don't remember seeing a right to marriage anywhere in the constitution.
Thirdly, there is nothing preventing a religious union between two men or two women, should a religion decide to endorse that union.
What I don't understand is the presumption that the legal status of a married heterosexual couple is completely sepearte from that of a gay couple. Isn't justice all about equality under the eyes of the law? I understand your point, that gay men still have the legal right to marry a woman, but why must the legal status of marriage only apply to unions between a man and a woman?
Is it not essentially the same thing? Ok, the word 'marriage' can't be used because it's defined as a union between a man and a woman, so let's just stick with 'legal union'. In order to create equality under the law, we must then rid ourselves of legal 'marriage', and create a law recognizing legal unions, between two men, two women, or a man and a woman, (or simplified to just a union between two consenting adults), and give all those who become joined in the eyes of the law the same legal status, with the same benifits currently reseved for only heterosexual couples.
See, it's not so much giving homosexual new rights, as it's creating equality under the law.