People have started speaking of "traditional marriage" as though there is some other kind. This is akin to speaking of "traditional arithmetic" or physics or biology. If the federal government is to have any power over marriage, it seems to me that it can be specified that for tax purposes a marriage is a union between one man and one woman; there is no need for any kind of amendment and none is desirable. It makes no sense to try to amend the constitution to cure all social ills.
It doesn't seem to have occurred to anybody - if it has, I haven't heard about it - that the battle to preserve heterosexual marriage was lost eighty-one years ago at the Lambeth Conference. At that conference in 1930, the Anglican church approved the use of artificial contraception, thus being the first Christian denomination in almost 2000 years to do so.
Here is the 1930 resolution. Link
Resolutions from 1930
The Life and Witness of the Christian Community - Marriage and Sex
Where there is clearly felt moral obligation to limit or avoid parenthood, the method must be decided on Christian principles. The primary and obvious method is complete abstinence from intercourse (as far as may be necessary) in a life of discipline and self-control lived in the power of the Holy Spirit. Nevertheless in those cases where there is such a clearly felt moral obligation to limit or avoid parenthood, and where there is a morally sound reason for avoiding complete abstinence, the Conference agrees that other methods may be used, provided that this is done in the light of the same Christian principles. The Conference records its strong condemnation of the use of any methods of conception control from motives of selfishness, luxury, or mere convenience.
Voting: For 193; Against 67.
By 1958 any reservations about birth control were gone. Resolution 115 made it strictly a matter of conscience. What this has to do with homosexual relations might not be so clear to modern people since there are very few if any that remember the events of 1930. The import of this is that it made sterile sexual activity. or "Onanism" acceptable and eventually the norm. Once this is the case, what (other than aesthetics) are the arguments against homosexual activity? If heterosexuals can engage in sterile sex, why not homosexuals?
Attempting to suppress homosexual unions and abortion while ignoring the taproot of the problem is like trying to put out a fire that has an inexhaustible supply of fuel and oxygen by lighting back-fires or spraying water on it. Cut off the fuel source and the fire will take care of itself.
Many in the pro-life industry try to fight abortion without ever mentioning contraception. The few groups that do bring up contraception are probably genuine, while many of the others are probably worried that if abortion is ever ended, they'll have to look for another job.
Several years ago, a woman I know who is very smart and opinionated was yapping at me about the dangers of homosexual marriage and how we had to fight it, blah, blah, blah. In order to shut her up I told her that "as long as you have contraception you're going to have homosexual marriage." I wasn't expecting her to agree, but she stopped abruptly and took on a very shocked look and said, "You're right."
I don't know what the remedy is since Protestants - other than a very few - have accepted contraception as though it were approved from Mt. Sinai, and most Catholic priests never mention it because they don't want any trouble. In a few instances, I have heard of priests being told by their bishops to stop preaching on the subject because they get too many complaints.
Passing laws is not the answer. The Catholic Church did not conquer and absorb the Roman Empire by passing laws. It is true that by the time of Theodosius the Church was in a privileged position, but this was after a long struggle against persecution. A step in the right direction would be to abolish any licensing requirements for marriage and return it to a strictly religious/sacramental institution.
C.S. Lewis suggested somewhere that there should be two kinds of marriage, a religious one and a secular one. If you want to have your marriage witnessed by the clergy you could do so, and if you wanted to be "married" by the government you could do that too, but the Church would not require a marriage license to perform the ceremony.
The old saying that "You can't fool Mother Nature" keeps showing its validity all the time.