If I have Sigarda, Host of Herons in play, whose rule text says.
Spells and abilities your opponents control can't cause you to sacrifice permanents.
And my opponent plays Remorseless Punishment:
Target opponent loses 5 life unless that player discards two cards or sacrifices a creature or planeswalker. Repeat this process once.
Can I choose to sacrifice my creatures, avoiding this effect due to Sigarda, or am I forced to choose another option?
You cannot sacrifice a creature to avoid losing life to Remorseless Punishment's ability. You can still discard cards instead.
One of the rulings on Sigarda, Host of Herons says
If a spell or ability an opponent controls instructs you to perform an action unless you sacrifice a permanent (as Ogre Marauder does), you can’t choose to sacrifice a permanent. You must perform the action. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Killing Wave does), you can choose whether or not to perform the action. If you don’t perform the action, nothing happens, since you can’t sacrifice any permanents.
