Because (a) it is a far more severe action involving the taking of a life and (b) it is not in the best interests of society-at-large to allow it.
This second bit... (b)... is usually the litmus test for whether something is allowed or not, over the long haul, despite an occasional flip-flop on legal standing here and there...
So depriving an American of, in this case, her life is verboten, religious rights and freedoms notwithstanding...
...what degree of harm can we rightfully allow? At what point is the amount of the harm a person would suffer from someone acting against them in the name of religion be small enough to make it permissible?
Tell us where that line of demarcation is.
Hell if I know.
Shooting from the hip, I would say...
We draw the line at Traditional Interpretations of Morals and Community Standards...
According to Western Law, homosexuality has been illegal for much of its recorded history, post-Antiquity, in most lands and jurisdictions and at most times...
And, of course, we brought those Traditional Interpretations of Morals and Community Standards along with us, from the Old World, and have been evolving our own 'flavor' of them ever since...
A sort of Differentiation of Species, in a metaphorical legal context, now that we have Western Law on ONE side of The Pond and another flavor of Western Law on ANOTHER side of The Pond...
Over the past few decades since the end of WWII, which largely enervated and emasculated Europe as a world power point-of-origin, the European variant of Western Law has grown increasingly dissipated and degenerate and emasculating to match...
Over the past few decades, America has resisted this downward slide towards degeneracy, but imperfectly, and homosexuals have done an excellent job of creating rationalizations and legal arguments and propaganda not only to advance their cause but to convince an increasingly large percentage of the Straight population to at least let up on them enough to allow them to make some legal progress...
Many folks have reached the conclusion that this process has gone too far, and that we are allowing our own 'flavor' of Western Law to more closely mirror the dissipated and degenerate and emasculated state of such Law in Europe...
So, they start talking about DE-legitimizing homosexuality as detrimental to society, to counter putting the rights of the 3% above the rights of the 97%... a state of affairs which simply cannot be sustained for very long.