Some interesting tips here from relationship counsellors Relate.

I have more initial meetings with people who are in this situation than you might expect.

The crush has acted as a trigger for them to seek legal advice to find out their options if they were to separate so that they can make decisions for the future.  

They may or may not have a feeling about what they want to happen with their long term relationship.  The crush may or may not have developed into something more serious. 

I'm happy to have these one off meetings, as I know they are appreciated by my clients.  They are also a good opportunity to sign post support, like counselling, if they feel they want to work on their existing relationship.