I'm not a native English speaker, but I'd like to try anyway...

If I'm so confident to win a bet and not to lose it, I'cant imagine that [punishment] will happen to me. That the [punishment] would happen for real (so I put at stake physical integrity).

How about if you write something like:
... to be so confident to win / not to lose... ... that they did not believe / did not expect / did not fear / did (or could) not imagine that the punishment could/would/will ever take place / happen to them?


Or maybe:
... they thought it would be impossible that ... [to lose / to get the punishment / the loss of physical integrity, which they put at stake]

Cheers!
Can't you p