The Church of Jesus Christ of Latter-day Saints teaches to respect, love and serve everyone, even those we may not agree with. Same-gender marriage has become legal in the US. However, it is not acceptable according to the LDS Faith.
LDS believe that same-gender couples should be free to live together if they so choose, and not be denied any medical decision making, insurance, employment, or taxation benefit that traditionally married couples receive. The only thing that sets civil union apart from marriage is the right to bare and raise children.
That's it. Children. That's the only difference. LDS believe that children have the civil and God-given, unalienable right to receive a genetic inheritance from both a male and female and to be nurtured, where possible, by a mother and a father. Same-gender marriage violates this right of children to be nurtured by both a father and mother and potentially will violate genetic laws as couples are seeking advanced genetic/fertility technologies to generate offspring.
Because of the violation of the rights of children and the genetic and social dangers this lifestyle will pose to society, it is not an acceptable practice according to the LDS Faith which wishes no harm to anyone. So, while same-gender couples are welcome to worship and participate with the LDS, they will be denied membership so long as they act contrary to the laws of God.
In the spirit of doing no harm, children of same-gender unions will also not be given membership until they are 18. The LDS Church will continue to teach traditional marriage as our gospel standard and we do not wish to subject children of same-gender unions undo stress when they learn the Church does not approve of their parent's choices. Similar policies apply to children of polygamous unions for similar reasons.