Equal rights will do no harm to marriage
Gay partnerships are about equality, not sameness. While the comparison with unmarried heterosexual couples is often made, the reality is that unmarried heterosexual couples have the right to become married if they wish, and to partake in the benefits of marriage.
Likewise Rónán mentions the case of the brother and sister - the comparison is not really valid as familial relationships confer rights similar to marriage such as next-of-kin and succession rights.