Obviously the question is a trap. As such, they're not going to torpedo their own boat playing along.
We already, through thousands of years of humanity, know that a male-female couple can raise children successfully. That is proven. We cannot say that they absolutely always will but that they can. Now, what do we know about a gay couple raising kids? Less. It's that simple. Because history is not on that side, it's newer and, thus, less proven.
In any scenario where the new option even holds even the potential for new risks and no new potential benefits, why even consider the new option?
For me, it boils down to something similar to what I posted in the other thread when races were brought up. As much as anything, it's just a matter of making things easiest on the kids. Do two kids of a single race face less scrutiny about the color of their parents than kids of a mixed family would? Possibly. Would kids who have clean new clothes get made fun of less than someone with older or more worn clothes? Possibly. The clean clothes and the single race family don't inherently make anyone better or worse but if it takes some drama out of the kid's life, the benefit is already identified. I'm probably stronger for having grown up in a family where clothes were one of the least significant considerations but would I have appreciated if we'd just had clothes like everyone else and I just learned that strength somewhere else? Absolutely.
That's what it boils down to, for me. There are more potential issues in the gay scenario than the straight one. Take a variable out of the equation and you'll always get a more predictable situation. Whether the variable is the most minute or world changing, it is a variable to some degree.