Most, if not all, would be aware that the Scriptures speak quite clearly against these lifestyles. So I'm not really interested in looking at it from that approach in this post.
The prophet John Taylor stated, ".. it takes a woman and a man to make a man." 1877 Journal of Discourses 19:245
For homosexuality to be a correct lifestyle John Taylor would need to be incorrect. Such a lifestyle proposes that a man can be whole with another man, or whole on his own. It would equally suggest that women can be lesbians and be whole on their own, also.
I'd again like to quote from John Taylor _
"Sisters, you are eminently constituted for this work. God has given you both the desire and ability to do it; you can enter into the sympathies of others, and you can better appreciate their feelings than we men can, and you are altogether more competent to minister in such affairs. Hence the Prophet Joseph Smith, in his day, organized a Female Relief Society.." Journal of Discourses 19:245
Looking at John Taylor's claim he is proposing women to be different from men (which I think none reading would dispute). He mentions that women are more into the heart issues. This doesn't mean men don't have a heart, as Jesus Christ is a man (for starters). But it means that a good woman is more that way than an EQUALLY good man, or a bad woman as to an EQUALLY bad man.
This then establishes that a man and a man relationship isn't as complete as a man and a woman, because the latter relationship is MORE capable of dealing with heart issues. Note that I have emphasised the word "MORE". This is to again say that I have not proposed men have no heart, only that women are better in this area overall.
Men are better in the cold logic area than women. This is helpful where the heart may lead to wrong decisions. This is not proposing that women have no logic. It is saying that a good man has more logic than an EQUALLY good woman, and a bad man has more logic than an EQUALLY bad woman.
So a woman and woman relationship isn't as complete as a man and a woman relationship either. It hasn't as good an ability to deal with logical issues.
Someone may argue, well what if my same sex partner is good, isn't that better than me marrying a bad person of the opposite sex? The point is to find someone of the opposite sex that matches you and grow together.
Gen 2:18 "And the LORD God said, It is not good that the man should be alone; I will make him an helper OPPOSITE to him." (UMC - Unauthorised More Correct version). Note the word "opposite", rather than "meet" as used in the AKJ (Authorised King James version). While the use of the word "meet" isn't incorrect in intension, the word "opposite" is a more accurate translation of the actual Hebrew word used. It means opposite or facing from the other side (according to Strong's Concordance). And it is appropriate to the point of obtaining a complete balance in a relationship.
So far I have dealt with the imbalances of each individual sex. But the other point I would like to bring out is that of what a marriage actually is. Is marriage a sex license, to feel accepted with God or society? Or is it for saying we will live together and share time together? Is it for saying we love one another so much that we are prepared to live under the same roof? These may be side lines to marriage, but aren't what it is.
Marriage was given for man and woman to produce offspring, and raise those children in an environment that would give the child complete balance and instruction within a secure relationship. If we add other good things to it, that is fine, provided we don't forget what a marriage actually is. It is because society has forgotten what marriage actually is that we have 8-year-olds having sex to gain experience, a "try before you buy" system, and misconceptions of gender.
I love my brother very much, and my sons. I loved Spencer W. Kimball. I love Heavenly Father and Jesus Christ. I love the latter two more than anyone else I love. I would love to go home and live in Heavenly Father's presence. Living with someone you love is wonderful. But I can't produce offspring with them. That love isn't what marriage is about. And sex must be reserved for that which it is naturally for _ having children. Whatever fulfilment a person may feel they have in these lifestyles is nothing to that which you can have in a correct relationship with the right person of the opposite gender.