Because many same sex marriages do provide stability and not all traditional marriages provide stability in any sense of the word. So in essence, this reward is not based on the behavior you claim it is meant for.
Furthermore, marriage is about the joining of two lives through a sacred or ultimate pledge. It is not about propagation. Though an important part of life, the lack of children does not invalidate or devalue a traditional marriage.
Finally if it is an all important goal of society to employ best method for raising the next generation of citizens, then it would enforce standards to that end and take steps against those who don’t provide such a setting. Are you calling for the government to come in and take away the child?