I will comment and say that a lot of the counter-arguments are just as bolocks as well. The comparisons to slavery, Apartheid for instance are very much stupid as they claim these arguments against. Yes there are legimitate legal concerns.
Even the supposed slope is inaccurate, the big push now in the states is for transgender "rights", and there was a case in the US, recently, where these unfortunately have been exploited. And the same kind of attitude is shown, expectedly against states like NC, that seek to actually think some common sense and safety into that issue.
Some other comments about the arguments (strawmans and the non-strawmans; and their supposed refuttal):
#1 is not accurate, no one argues along those lines generally speaking.
#2 is also not accurate, you would find that most people care about this issue, because it affects one of the fundamental things in society, marriage.
The refuttal to #3 expresses a common misunderstanding about the fact that marriage isn't just about love, and certainly is not the sole quantifying factor. So more of an agree to disagree.
#4 is actually partially true, though considering that recently GetUp has shifted the issue lower in priority.
#5 - There are a lot of things that are in nature, so what?
#6 - The science is inconclusive, I know that many homosexuals would feel a certain way about it. But experience does not negate the fact that there is no conclusive evidence on being "born this way". The science does not conclude favourably in either way.
#7 - Polygamy is the next logical step. Clearly this person limits it to consent.
I will note that in some states in Aus (ahem Victoria), the consent age is actually 12*. So...? But the next step in the slope is actually transgender rights, we can observe that from the US/Canada. (*with a bit of other rules attached)
And you get the point...