"The Big Lie" With Silently Spoken Answers

Lie #1 -

"When I want to do the singlespeed thing, I just don't shift"
No. You will.

Lie #2 -

"When I want to do the fixed-gear thing, I just pedal all the time"
No. You won't.

Lie #3 - (internal dialogue)

"I'll set up the flip-flop hub with a fixed and use the freewheel for a bailout"
Once you go fixed, a freewheel seems like a cheap imitation. You will be selling the freewheel sprocket on ebay within 3 months.