If Christians really want to "defend marriage" why aren't they trying to amend the Constitution for adultery?

Because they don't have the guts to take on heterosexual sin, which is rampant via adultery and fornication, and there's no $$$ in pushing heterosexual adultery. It's much easier and brings in much more $$$ for false prophets when they pretend like eradicating gay families from our society will magically fix the many problems in heterosexual marriages. Anything that fails 50% of the time needs a lot more help than simply bashing gay taxpayers can provide!