Their decision, Printmiss - not for me to judge. My grandson lived with his wife for seven years before they married last year. They have two little girls and are very happy. My grand-daughter has lived with her fiance for about the same length of time and they also have two little girls. They will marry when they can afford it (and the girls can be bridesmaids.)
Another grand-daughter (I have five) is living with her fiance in Nelson, New Zealand while she finishes her nursing degree. They have been together since they were 14 and he moved across the world to be with her. They will marry when they are ready - she wants to get her career started before they do and also to give her family and friends time to save up, as at least half of them are going to have to travel 12,000 miles.
With about half of marriages in the UK ending in divorce, it is no guarantee of a lifetime together. (Although I do think that married couples are more likely to stay together, statistically, but the figures include couples who have just lived together for a short time).