I’ll take a stab at that.
I see two issues. One of them is the slippery slope argument. And it is a valid one. For example, you can trace the elimination of legal gender distinction in marriage over the past 100 years or so. Or you can go back further to the abolition, and trace the civil right to enter the contract of marriage from the end of slavery onward. As you progress on the historic path, you see that while at one time, there were very clear legally defined gender obligations - a clear distinction (for example a husband could sue for divorce if his wife would not relocate with him when he decided the matter, though the converse was not true until quite recently). Today, there are no legal gender definition of duties in a civil marriage contract. Hence, it is an equal rights violation to discriminate based on gender as to who may get married. This is a clear example of the slippery slope. While some would call it a slide into moral depravity, others would call it progress. It depends on your perspective. So that is the first issue, and why the slippery slope is so important. I see this as a strength of Catholicism because it does recognize the historic imperative.
There is another issue with I perceive, and I hope that I won’t get beat up over this. My perception of Catholics is that there is a skewed moral calculus as to the gravity of sin. For example, masturbation is a grave matter. So is adultery. So is child abuse. It all gets rolled up as one ball of yarn, illicit sex. So, my opinion, is that it becomes more difficult to distinguish the gravity of offenses, when everything is such an important matter, and pretty much equal. A mortal sin is a mortal sin, whether it means abusing a child or masturbating and falling asleep. In the calculus of redemption and forgiveness, it is all the same. I believe that this is part of what has impeded the Church in cleaning up some of the issues which hit the news. There appears to me to be a failure in ability to weigh the relative moral weight of sinful behavior, because it is all essentially equal. So, when you mention gay sex, then it is equated with any other sexual offense in the catalog. I see this as a weakness in Catholicism, for its inability sometimes to distinguish the grey areas, and also it impedes social progress in some areas.
Therefore, if you allow gay marriage, then the next step is bestiality, by both arguments. It is one more slide down the slippery slope on the one hand. The two behaviors are morally equivalent on the other.