Slippery slopes are a fallacy, so please don't use that in an argument. Here it's not a case of slippery slope, as in, it must lead to something else in the future, but a present correlation, and a strong one at that. It's like in my unlicensed drivers example, it's not a slippery slope argument that they'll cause accidents, it's actually an existing fact that they cause a disproportionate amount of accidents (given their percentage of cars on the road) if they drive.
A slippery slope is more like "if you disallow trans in women's bathrooms today, you'll then want to arrest them." Like another esteemed forum member was doing earlier in the thread. Discussing things that have already happened is not a slippery slope.
As for Thermal's argument -- and to be clear, it's just about the argument, not the person -- you don't need that. What he's trying to do is the usual "if you don't let men into women's spaces, you're a transphobe" canard. Although admittedly, he's doing it more eloquently and subtly than the usual crop of "progressives", so kudos for that. But that's all that that blurring the lines is arguing about: trying to force framing it as somehow caring about your or your family's safety is being unreasonably against the trans, or at least a jerk to them. That's all.
But yes, we're back to the usual rolling back all arguments presented before, and you being a transphobic jerk if you disagree.