Have a sit down with B and find out what marriage means to him, define it, what makes it important. Is it the legality, the piece of paper, the old implied "till death do us part"? That might help with figuring out what "angle" to take on explaining why marriage (legal document) is not as important as what is behind it (the emotions and commitment).
Although I/we have not had the experience yet, this is about what I am hoping to find for my family. What I mentioned above would be an important conversation to have with a possible "second hubby". I only use first or second as a measure of time really not degree.