Marriage is a human right, but marriage is between a man and a woman. In the case of the heterosexual couple there is a vast industry devoted to preventing pregnancy just so the sexual activity through which romantic love can be expressed can take place without the need for any responsibility to be taken or commitments made should a pregnancy result. The idea that gay couples shouldn't be allowed to marry because they can't procreate is extremely popular. Opponents of gay marriage appear to recognize this. However, thus far conservatives have failed to explain how or why treating gays like fully equal citizens and human beings is incompatible with anyone's religious liberty.
It is like arguing that the function of a cupboard is the same as a refrigerator because some refrigerators don't work, but can be used to store things, so it is the same as a cupboard.
Those who possess that authority and power and who have used them to create their identities are thereby threatened by the perspective changes. On the other hand, if any of the premises are false, then the conclusion is false, though the argument is still valid since the form of the argument is valid. Defining marriage in terms of sex begs the question of how we define a person's sex in the first place.
The love between a man and a woman has the possibility of not just intimacy but also of the procreation of life.