God does not sanction same sex marriage. Leviticus 18:22 – Thou shalt not lie with mankind, as with womankind: it is an abomination. God has a certain order in when it comes to sexual relationships. God meant for a man and a woman to be in a sexual relationship. 1 Corinthians 11:9 – Nor was the man created for the woman; but the woman for the man.
The man and woman are able to procreate and God set up the institution of marriage to raise those children in. That way they grow up in a balanced environment. When children grow up in same sex parent households they do not have that original model to reflect back to.
One reason God does not condone same sex relationships is that is goes against the purpose of the male and the female to procreate. This traditionally cannot be done with same sex relationships.
Spiritually speaking, same sex relationships garner judgement from God as they go against His natural order. God created man and woman to work in unision within the confines of marriage to rear the children.
1 Timothy 1:10 – For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and there be any other thing that is contrary to sound doctrine.
God has standards He has set in place. He wants people to be holy as He is holy. 1 Peter 1:16 – Becuase it is written, Be ye holy; for I am holy. God has laws that He would not have us ignorant of.