It's much easier to justify if you keep coming up with ever-increasingly niche descriptions for them so there is only really ever one bike in each category, for instance:
Folding bike
Tandem
Single Speed MTB
Modern MTB
Modern road bike
Winter commuter
Summer commuter
Winter touring bike
Summer touring bike
Lock-up bike
Child-carrying bike
Cross bike
Rough Stuff bike
1980s American MTB
1990s American MTB *
1980s UK MTB *
1990s UK MTB *
1970s UK road bike
1980s UK road bike *
1990s UK road bike *
1980s American road bike
1990s American road bike
1980s Italian road bike
1990s Italian road bike
* indicate more than one currently in this category, sub-categories are yet to be defined.
I also find that keeping some unbuilt takes up less room and then means you can bunch them all in together as 'projects', thus never truly admitting the extent of the -problem- collection.