Though they are highly likely to have been related Hopkin/Hopkins is not a rare name and they could have been no relation or very distantly related.
Sometimes people oppose things when things have gone wrong in other generations e.g. a relative marriage may not have worked out which may be why it is opposed in a different generation.
As far as mainstream Christianity and the British law is concerned there is a list of relationships which are allowed and a list not allowed in terms of marriage, I think, but would need to check, that marrying a first cousin is about the closest relative one can legally marry in both Christianity and by law. The C of E (and other mainstream brands of Christianity) in such a close commmunity would only have allowed marriages to take place if they fell within what the Church and the civil law allowed and there would have been harsh penalties for crossing over the well defined lines/"bounds of consanguinity".
Another factor, as well as the very close rural communities in which they lived could be religion e.g. RCs marrying RCs, Quakers marrying Quakers,Jewish people marrying Jewish people, Methodists marrying Methodists, and would therefore restrict their choice further. In the Forest of Dean language group could have been a factor, some people were Welsh speaking and may have therefore married other Welsh speakers or bi-lingual people rather than those who spoke English as their only language, which would restrict choice further.
Depending on the era in which people lived there would also be an element of arranged marriage, not necessarily done formally but by community expectation and pressure from those around them to marry a certain person or someone from the same class. Certainly formally arranged marriages were very common in Tudor times but also at other times too.
In my family there are examples of people marrying relatives, not necessarily as close as first cousins, e.g. relatives of step relatives marrying; marrying the brother of the mother's second husband (which made her mother also her sister-in-law and her step-father was therefore her brother-in-law, her half-sibings were also her nieces/nephew through marriage), etc.
I don't know where you are living now but if you are abroad, you might be interested to know that if any of your family were from over the border in Wales that the number of surnames there was relatively small. It is therefore quite common for a Jones to marry a Jones; or Mr. Evans marrying Miss Evans and a Morgan and a Morgan without them being related (or only related by many distant generations). I have found looking up Welsh relatives with common Welsh names is challenging for this reason.
Also in Wales it was traditional to call people by their father's name, e.g. Evan Ap Evans reduced now to Evan Evans, or Thomas son of Thomas being Thomas Thomas, Owen Owen, Morgan Morgan etc.