Cookie Jar

A[n] argument that has always been a bit weak has been the attempt to minimise the extent to which allowing same-sex marriages will change the definition of marriage for straight married couples.
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It's a big deal when social institutions change this way, and if conservative heterosexuals feel their marriages are affected, they're right, even when the way they phrase their complaints is wrong.
And now on to polygamy

I would say that this is badly phrased, almost to the point of worthlessness. A better away of putting it would be as follows:

[I]f conservative heterosexuals feel their marriages are affected, those feelings are valid, even when the way they phrase their complaints seem personal and subjective to others.

There is a difference between acknowledging the legitimacy of an emotion, and stating that such an emotion is therefore grounded in a factual circumstance. I have been unable to find any evidence of a change in heterosexual marriage beyond the loss of an exclusive privileged position for married couples. To use an example that parents may be familiar with - It is possible to accept that a child's hurt feelings at another child receiving a cookie are valid - but this does not mean that the child's argument that they are injured by the other child having a cookie - when they also have one and are not being asked to divide it - must be taken at face value. (And note, I don't intend to imply childishness on the part of heterosexuals who claim injury - the example is simply a convenient, and hopefully, accurate and easily-digested, analogy.)

As the meaning of the phrase "all men are created equal" was expanded to include non-landowners, women and non-whites, there WAS, on the other hand, a significant, factual change in what it meant to be a landed, white male - the loss of total control over the Republic. When the franchise was legally limited, those with it had the sole power to select representation. Others were subject to what the founding fathers claimed they had specifically fought to end: taxation (and being subject to a whole host of other legal circumstances) without representation. The end of limited access to the vote meant that circumstances were now different, as the once exclusive club of voters was now subject, at least potentially, to having their interests challenged, and maybe even damaged, at the ballot box by others who previously had only the choices of accepting what they were given, leaving the country or being criminalized. Straight married couples faced no such status alteration - and I have yet to encounter an argument for anything else that they lose in the transaction that doesn't transform marriage into a strange sort of zero-sum game, or an argument over a word.