By getting the pet spider, they are accepting the presence of a spider. Meaning that they do not find the presence of the spider unacceptable by the very definitions of the words. If you entertain yourself with depictions of sexual assault, you are accepting the presence of sexual assault (accepting here means saying "it's acceptable", or appropriate, or not bad enough to utterly reject). The same logic applies.
If spider and sa are interchangeable to demonstrate the mechanim, let's use another word.
If you entertain yourself with murder by reading novels, playing games or watching shows about it, you accept the presence of murder. It beomes more appropriate, to the point where you would not utterly reject it.
But murder is too harsh. Let's take a less severe crime. Like theft and breaking the speed limit.
If you entertain yourself with content around theft and speeding, like gta or fast and furios movies, you accept the presence of those things. They become more appropriate, to the point where you would not utterly reject them. Stealing a car? No big deal. Speeding? Bah, they do it in games and movies all the time, so I can do it too. It is appropriate behaviour after all.
Any increase in speeding and car thefts after release of a new GTA or a new movie? You could see such things in statistics and back up your line of thought about acceptance from seeing things to accepting them and finally imitating things. It is minor crimes, compared to sa, so the barrier to do them in real life should be lower, and it is a lot more popular, so a lot of people were exposed to the bad influence.
Copy cat crime from fiction does not exist. It would be easily prooveable, and someone would have done so already and they tried and tried. Copy cat crime from actual crimes sadly does exist. The number one factor for shool shootings is to report in the news about a school shooting.
Doing a crime in the real world has a barrier. You propose a mechanism that lowers that barrier, over "accepting" that crime's "presence". By seeing that crime in a fictional setting no less. In short, you take away the ability of adults to distinguish between reality and fantasy.