There is research that strongly suggests that sexual preference may be at least partially genetic. There are examples of homosexual behavior in God's animal kingdom. Homosexual relationships are not natural. Same sex relationships are clearly in the minority, there are more than enough families to produce offspring. Same sex relationships are intended to be based on the same principles of the love covenant of faithfulness than the traditional marriage relationship.
Marriages fail too.
Same sex relationships do not seem stable enough to qualify for the kind of covenant relationship marriage is based on. Blessing same sex unions will restore and heal the shattered self-image of homosexual brothers and sisters. Jesus was always concerned about healing and reaffirming the validity of people around him. If same sex relationships should turn out to be against the intentions of the Creator, blessing same sex unions may be harmful to homosexuals, emotionally and spiritually. Christian family values should not be based on sexual preference or certain roles, but on the properties of trust, love, commitment, etc.