This fallacy states that because something is natural, then it is good for you. That can be the case, but it’s illogical to apply it in all situations. For example, opium is natural, it comes from flowers and you can refine it to make heroin, but it isn’t a good thing for most people to take it.
One example that causes confusion or pain for many Forever Alone is the idea that a relationship is natural, that most people do this and our genes and biology drive us in this direction. But of course, the evidence is that some relationships are absolutely toxic and best for no one, and there are perfectly happy people who choose to never have a relationship.
I think the biological arguments also come under this fallacy. There is a theory of psychology called evolutionary psychology and the idea is that you can explain all human behaviour in terms of genes and adaptations. So in evolutionary psychology, evolution is driven by female sexual choice, and they choose traits (genes/genetics) that are good for survival. So in the past, from the stone age, women have chosen men who are muscular, healthy, tall, strong, intelligent etc. and although there is a lot of variance amongst human, there is an overall trend towards the species becoming taller, stronger etc.
So women tend to choose men who are sexually successful. I can confirm this. You can have a lifelong dry-spell and once you bag one, they are lining up! But the point is, the desire for a relationship is deep within us, within our DNA, because the species wouldn’t survive if we didn’t have it, so it’s a natural drive and is good, and if you don’t have a relationship, then there can be a feeling of failure, which is a fallacy of course, because not all children are born from long-term relationships. There are (many) single mothers. There are many happy people who are not a couple, this is just the way it is.