Prevent Thunar "Open with" to permanently change the default programme

[Lorenzo Sutton]

Since a few versions I see this behaviour in Thunar: if I open a file 
with the Open with menu and select a programme which is different from 
the default one, that 'new' programme becomes the default one. For 
instance I have most audio files opening by default (i.e. double click) 
with smplayer, if I once I open them via "Open with", say, in sound 
converter, then (Thunar?) sets sound converter as default.

Any idea on how I can prevent this?

Thanks
Lorenzo.