Don I think you are running into the same issue here, and I have the same objections. Justice is no more absolute than virtue is.
But I want to get back to your original statement to see if/how you have modified it before I respond further.
You said: "People who take pleasure in what the average human would find morally or ethically repugnant aren't living according to Epicurean principles and so we would have reason to intervene and attempt to get them to change. Just because they are feeling pleasure doesn't make their life choice-worthy. I wrestle with this, but the more I think about it, the more I'm coming to these conclusions."
When I gave an example of a known issue where "average humans" find certain harmless acts repugnant and questioned if that would make you declare the action automatically not Epicurean, you couldn't go there, so you started talking about justice.
Is your new assertion "People who take pleasure in what a just human would find unjust aren't living according to Epicurean principles and so we would have reason to intervene and attempt to get them to change. Just because they are feeling pleasure doesn't make their life choice-worthy. I wrestle with this, but the more I think about it, the more I'm coming to these conclusions."
Have you substituted "just" for "average", and "unjust" for "repugnant"?
If so, as Cassius has explained, there is the same issue. People have some commonalities in what they consider just, but there are also significant differences. Lol, contract negotiations would be so much easier otherwise! There are no absolutes.
I would also say it's not correct to label someone taking pleasure in _anything_ as not Epicurean. Remember that for me, taking pleasure means including all the consequences including future effects. However, if their pleasure impedes mine, I'll certainly make an effort to stop them!
Btw, it's 100% natural for humans to establish taboos, unjust or not. These happen even without religion. It's not supernatural-- it's a real phenomenon we have evolved to enact. I notice that people tend to label things they don't like as unnatural, but there's no grounds for that here. Also, Epicurus doesn't appear to use the word "natural" to mean innate.
I can see it makes you very uncomfortable to confront the lack of definite moral standards apart from individual pleasure. I think that's what makes this discussion relevant to where it started, because that's exactly why people cling to the fixed virtues in Stoicism rather than to pleasure.
They do not want to say they endorse a philosophy that could conceivably lead to a person making choices for their own pleasure which harm others-- but this is in fact _inescapable_ in a reality-based philosophy. The moment they start putting their preferential behavioral constraints on others as if there is something magical about their own morality that will make their choices work for everyone, they have left the material realm for wishful thinking.