This is not a good retort.
It's point-scoring strawmanning against gay marriage opponents, aimed at other gay marriage supporters, for internet points.
To provide an analogy for someone to laser focus on while ignoring my overall point;
"You think leaving frying pans unattended will lead to deadly fires. This just means you think ALL fire will kill people."
Inherent in the original position is the logic that gay marriage, pedophilia, and bestiality are all different from "normal" (straight) marriage.
A better argument; "Consenting adult humans are not treated the same as animals and pets."
Or "Plenty of places have legalized gay marriage. How many of them also legalized bestiality or PDFs?"
Or "So you're saying you don't have a problem with gay marriage itself, you just think it will lead to something else that's bad?"
But nope, they just went for the most ridiculous strawman imaginable.
You can think the argument right or wrong, but ignoring what they actually say so you can have a "So you're saying..." moment for your ingroup...is not really productive.