Good God..  Prenups!  lol.  Well I could understand if the SO wants to keep their property especially if it's before marriage and they have a decent fortune (even if it's a nice house or some collection).  If the SO agrees, then you know they don't care much about the money. 

In any case, if the SO didn't give a crap about you and your bitcoins, keep them for yourself .  I tell my wife about it but she doesn't think much about it just as long as I'm careful with what I'm doing.  In the event of something happening, I'd still give half to her. 

Now in court, that's a different story .  If you got married and accumulated bitcoins afterwards, half belongs to her.  But who's to say you ever had an account .  If she finds out you filthy rich after a few years and the value of bitcoins rockets up, you'll sure to be getting a letter from the courts to review your finances.