Surely the definition of being a husband or wife is that you are married - and being married is a legal thing.
Unmarried People may well have a relationship where they feel like they are married or where they are as close or closer than people who are married, but this does not make them married or husbands or wives.
I understand that many people are in long term, often lifetime relationships. Many people view such relationships as absolutely the equivalent of marriage. They might well be in terms of the feelings and commitment between those people, but they are still not the SAME as a marriage, because the legal aspect is missing. People often think the legal aspect of marriage is unnecessary or just a piece of paper etc - however, the fact remains, that a marriage has a legal aspect to it which isn't there in non- married relationships.
I don't think believing unmarried people shouldn't refer to themselves as husbands or wives is intended as a slight to them, or a criticism or belittling of their relationship at all - it is just a recognition that the state if marriage exists and not everyone has chosen to be within it.
Personally, I find it odd that people who have not chosen to marry, then refer to themselves as if they were married. I find it especially odd when people who seem strongly against getting married do it. It says to me how strong the historical ties to the institution of marriage actually are, however much society says marriage is outdated and no longer necessary or relevant. Many people have chosen not to be married (or perhaps would like to be married, but ding themselves in relationships where marriage isn't on the cards) and somehow feel that having a 'DP' or 'boyfriend' is seen as a lesser or inferior relationship. Despite the claims of society that cohabiting is the same as marriage, people still seem to feel it isn't. It could be that this is because we are really only about 40 years beyond marriage being absolutely the norm and it simply will take longer for society to truly accept this idea, or that intrinsically into the long term,people will feel there is a differnce.
I simply think it is incorrect for people to refer to their husband or wife if they are unmarried. Legally they don't have one. Having a husband or wife isn't about the strength of feeling between 2 people - it is a legal thing.