The biggest problem is where this presupposition defines marriage, which boils down to "two people feeling emotionally satisfied with one another." The idea is that if two people of the same gender feel satisfied in one another, then why shouldn't they be allowed to be together and even marry? It's the classic "who cares so long as the person's happy" fallacy. If you attempt to qualify marriage in any way (the interaction of the genders, the ability to procreate and produce families, the need for a mother and father figure on the child, etc.), they find all that unnecessary, and toss it out. They demand that we make the definition of marriage be the satisfaction found in a union between two people.
Of course, what happens when this same standard used for homosexuals, and this same definition of marriage, is applied to other areas? What of "intergenerational" relationships, such as between a 50-year old man and a 15-year old boy? If it is entirely consensual, and the two feel emotionally satisfied, why is it wrong? If a brother and sister fall in love and decide to marry in an "interfamilial" relationship, and the two feel emotionally satisfied with one another, why is it wrong? Shouldn't any and all examples of two people feeling emotionally satisfied be permitted and accepted by the government under the law? This isn't even covering the issue of why, if it's possible for more than two people to feel satisfied together, polygamy should be illegal.
When these issues are brought up, usually two responses are given:
1) Straw men. People will accuse you of equating homosexuality with pedophilia, incest, polygamy, etc. The fact is, this isn't what we are doing - no one is saying that all homosexuals are pedophiles and vice versa, or that homosexuality and incest are the same thing. What we are attempting to do is to show the shallowness of this marriage definition, and just how weak a position it is by taking the same presupposition and presenting it in a similar situation. This is especially important as, at the time of this writing, those who promote "interfamilial," "intergenerational" and polygamous marriage make the exact same arguments made by those who promote same sex marriage. Why does the position of "equal love; equal rights" not work for them?
2) Inconsistency. The person who just a moment ago did not like you qualifying marriage...will now begin to qualify marriage. They might bring forward reasons why an elder and a minor shouldn't be married, or why siblings shouldn't be married, or why a marriage shouldn't involve more than two people - and some of their reasons might be perfectly valid and legitimate. The problem is that they are holding "interfamilial" and "intergenerational" marriages on a different standard than they are same sex marriage. On the one hand, they deny that you should qualify marriage; on the other hand, they affirm you should qualify marriage. In this case, even the same sex supporter - though unintentionally - realizes that a definition of marriage as "two people emotionally satisfied with one another" is a feeble one at best.
From this comes an obvious conclusion: the idea of "equal love; equal rights" and "love equality" is simply a sham. It's simply an emotional slogan for the cause of same sex marriage, so that its supporters can cry it out and feel as if they have done something grand. Yet they do not really approve "equal love; equal rights" because they believe that if a brother and sister have fallen in love and desire to fulfill their dreams of marriage - perhaps even adopt a child and raise it with love - then it should be opposed by the law. If a 25-year old and a 14-year old fall in love and desire to begin marriage, they oppose it and believe statutory rape laws should stand. It is clear that even in their minds, not all "love" is equal, and not all "love" should be given the same rights other "love" is given.
It is popular for those in the same sex marriage camp to compare those who oppose it to the racists of the 1960's and those who opposed interracial marriage (a fallacy I've examined before), but even in their case this same analogy can be lodged their way. When they support the marriage between homosexuals but deny marriage to other groups who, by their own logic appear perfectly legitimate, then they are no different than those in the 1930's who opposed the Nuremberg laws against Jews in Nazi Germany, but approved of the Jim Crow laws against blacks here in the United States. They are no different than someone who might approve of their child marrying an Asian person, but not approve of them marrying a black person. Already there are voices in this country, seeing the privileges given to homosexuals, who are beginning to ask that "interfamilial," "intergenerational" and polygamous marriages be permitted as well. No doubt in decades to come, many who supported same sex marriage will find themselves opposing the governmental affirmation of "interfamilial" marriage - or any other kind of distortion of how even nature has always defined marriage and true union - and they too will be called racists and intolerant bigots by the coming generations who have been raised to emotionally respond and parrot popular arguments, all for the sake of a new wave of public opinion.
Of course, we know why mankind is inconsistent, and why they desire to support their favorite sin, even in the face of an inconsistent worldview.
And since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done. They were filled with all manner of unrighteousness, evil, covetousness, malice. They are full of envy, murder, strife, deceit, maliciousness. They are gossips, slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents, foolish, faithless, heartless, ruthless. Though they know God's righteous decree that those who practice such things deserve to die, they not only do them but give approval to those who practice them. [Romans 1:28-32]The only reason we have not yet seen greater immorality find greater approval in our culture is because God has thus far restrained it. If we inspire more judgment, more judgment shall come. Man's natural state is open rebellion against God, and their denial of Him will come in many forms, whether it be in false religions, apostate churches, or embracing sins such as this. I saw someone on social media say the Supreme Court made the right decision despite God's intentions. Yet God is still in control - indeed, the spreading of sin is by God's sovereign will, so that His judgment may be true and His cause glorified (cf. Ro 9:22-24). The increase of sin is not a sign that God has failed, only that God has judged, and is bringing further judgment.
How should we take all this? We should be praying for the repentance of friends, family, and neighbors. We should be praying not that they would be correctly aligned in politics, or that they would understand this one issue (for your opinion on homosexuality does not save you), but rather that they would come to an understanding that they themselves are sinners deserving of judgment. There is a natural law, even if they deny it, and it is God alone, not the Supreme Court or any other higher court, who will have the final say on what unfolds on the day of judgment. On that day, all men will be held accountable for their sins, but only those purified by the blood of Christ will be deemed innocent. Regardless of the state of our society, we should all ponder the state of our souls, and remember that no matter how many years we live, eternity shall be a long time. Let us remember that as we look upon society and those who oppose God - and let us not forget that once, too, we were like them. God bless.