"what would it hurt, just once?"

If it were just once, perhaps not much harm...

But once has a tendency to become:

TWICE,

Then again, and...

Just once more

A single stick or rock in a river
will not do much to change the course
of that river.

But the rock sinks to the bottom,
trapping silt and sediment, building up,
catching sticks and other barriers,
eventually causing the river to bend away from its set course.

Once tried, worldly enticements
sink into the rivers of our souls,
catching other worldly enticements...
building up barriers to the spirit,
changing our set course.