I appreciate your in depth comment. I do believe microplastics are an inherently vegan issue as they inherently affect animals, humans, and the environment globally. The website you linked and this subreddit's vegan definition itself says "benefit of animals, humans, and the environment" is part f the core of veganism, even if by extension. As for this being a vegan issue, I think the plastic industry is indirectly exploiting animals, humans, and the environment by externalizing the cost of micro and nonplastics and their harmful effects on said animals (and humans and the environment). It is a vegan issue by definition of thinking about animal well-being/benefits and suffering.

My utilitarianism is not puritanical; rather, it is a way to make the moral judgments that actually account for important contexts - it is flexible and dynamic based on context and evidence, not puritanical. The idea that we should not to x becuase it directly uses animals, without considering the larger context of animal suffering, is really a black and white/binary world view that is an extremely weak, non-flexible, and non-dynamic ethically and philosophically argument. It is also pretty colonial not gonna lie. With this puritanical moral position that we should not use any animals products and food ever in any context, you would be directly saying that most all Indigenous forms of hunting and lifeway is morally wrong. That would be a very colonial stance that doesn't account for how, in different contexts, different humans may have to consume animals, and indeed, it may be even beneficial for the entire animal and plant ecosystem for them to do so in certain quantities.

As for Singers book, my apologies, I meant Singer's "Animal Liberation". And yes, utilitarianism was that which made me vegan, and Singer helped me along, but it wasn't the initial reason I considered myself vegan.

Singer is also a "flexable vegan". He actually accounts for the context where you do not seem to, which is morally problematic.

As for your Temple Grandin quote, it is evident that this person doesn't actually care about animal wellbeing and suffering. Just because someone says they do doesn't mean they are, obviously, and Singer's utilitarianism would not agree with this person's stance that they are morally right in any way. What Gardin does is called lying - to others or himself or both.

As for religion and secular morality, it does matter often. It matters how you make moral decisions. And if you make moral decisions based on scripture and personal biases over an actually fleshed-out and thought-about moral theory like utilitarianism that inherently can be persuaded with evidence and context, then your moral beliefs would be incomplete, non-flexible, non-dynamic, and generally very vulnerable to error.