Up until the 20th Century, forbidden marriages in Church Of England parishes were determined by the original 16th century "Table Of Kindred and Affinity".
This has been changed a couple of times in the 20th century, the last being in 1949 I think....
Google it and have a look. (There are some simplified versions of the original which are easier to read).
This is quite a good site.
http://www.genetic-genealogy.co.uk/Toc115570145.html#Toc115570255He would not have been allowed to marry his niece.
I have plenty of 1st cousin marriages and I thought that would be banned in the updated tables but no...it is not mentioned
There is also a table of genetic risk but I couldn't recall where I saw that one.
The most common reason for not finding a marriage is that divorce was very ahrd to obtain and very expensive.
So a lot of people lived as Man/Wife without ever getting married.
Some get married after living as man and wife for many years and they learn their first husband/wife has died, thus removing the need for divorce. So sometimes it is worth searching across all years rather than just before they appear in the census together.
Some risk the bigamy laws and marry without a divorce. Rarely do I see them do this in the same parish as their first marriage so it is worth looking all over the country.
Black Holes happen when God divides by zero.