The current fuss over David Laws shows that defining relationship becomes difficult as soon as you move away from some kind of legal registration, which can give objective evidence that a connection exists - i.e. a certificate. Mr Laws' relationship wasn't a civil partnership, they didn't share bank accounts, they didn't have a public shared social life, and the information revealed thus far indicates that he didn't stand to benefit personally from any of the payments made as rental.
That leaves those investigating this case with a problem - where is the threshold if it isn't the legal registration of it as a marriage or civil partnership. It's not about the quality of the interpersonal relations - there are plenty of examples of bad marriages, and very good relationships between people who are not married/civil partnered. The issue is how can a relationship be evaluated objectively without the existence of some form of legal contract between the partners.
For example, two people could co-habit in a way that was clearly a lodger/landlord relationship. That wouldn't run into trouble. The landlord and tenant could be friends and still there needn't be a problem. People have sex with each other without wanting their relationship to become a partnership. So what defines David Laws' relationship, which may or may not be a 'partnership' in the eyes of Parliament?
In the end, it will be a matter of opinion as to whether this relationship crosses the line of the Parliamentary rules, and there will be a degree of subjectivity involved in the judgment. David Laws didn't want this relationship to become public, so he treated his rent on a business basis. The taxpayer didn't pay more for his flat than others, and his expenses overall are relatively low. He hasn't benefited personally, and he could have made a lot more money doing something else. But he did conceal with a degree of deception, and he has fallen into the grey area of when is a relationship a partnership. Some clarity is needed for everyone's sake, but I fear the media will judge him long before committees on MP's behaviour come to a mind on it.