I think you're right...he probably hears that "threat".
Would it be good/helpful to ask him what he thought would be fair or right and why? Maybe one by one over coffee in a public place where he might be able to better manage his feelings.
If you don't want to ask for the lump sum, maybe that is something you could discuss in lieu of him selling the bike.
Retrouvaille had a point in saying feelings are not right or wrong. He feels attacked or hurt. Even if that is his fault, it isn't wrong that he feels that way. It is wrong that he can't see his responsibility in the mess. One day, he'll wake up and say, 'whoa...what was I thinking', but that day isn't there yet.
Are you able to use some empathy-like statements when you discuss these further...like, "You seem very upset. I know the bike meant a lot and I don't want you to have to lose it. I just want a fair settlement if you are sure you do not want to be married anymore. I'd rather we work on the M." (talk about you instead of blame, accept his feelings, offer reasons that acknowledge your need)
