Myth #1 is utterly rediculous. It takes genuine maturity, responsibility, and commitment to make marriage actually work, i.e. to go ahead and actually bother despite the price tag assigned to being married. Seeing the declining attitude that people seem to have toward actually growing up and getting a life, it’s rediculous that something as simple as a few people having what they deem to be marriage would have much, if any, effect on the maturity of the populace, which has declined, and has little incentive to actually rise, given the decay of the general economy, decline of religion, and the rise of the welfare state.
The idea is that by redefining literal marriage to include same genders - and which are rampantly non-monogamous - it effectively redefines it to include the joining of anything, and thus, like grade inflation, it reduces its uniqueness and the meaning of marriage and morality, thus promoting non-committal cohabitation.
Behind this is the implicit religious underpinnings of marriage, and which esp. in Scripture Christianity, only and specifically define marriage as being btwn opposite genders of humans, and condemn homosexual unions by precept and in principle, by design and decree. That is good enough for me, while abundant evidence testifies to the deleterious nature of homosexual activity.
Your argument is void of any logic. Adding homosexual sex premised couplings does nothing but cheapen real marriage. Libertarians need to stick with fiscal issues as they are morally clueless and can not defend what they can not even hope to understand.