Sorry, but you're way behind the times. Many gay and lesbians couples have children today through a variety of methods-in vitro, sperm donor, surrogate mother, adoption, already had the kid before went gay, etc.
And there are a lot of reasons for marriage other than procreation
And there are a lot of reasons for marriage other than procreation.
There could be any number of reasons why people may get together and formalize their arrangements, but procreation is certainly the main reason that the state regulates marriage.
One man and one woman marrying and producing children and forming a family unit is the pattern that the state has a compelling interest in preserving. The state does it by supporting the traditional institution of marriage. I ask again: what is the compelling reason to change this state of affairs?