Can you react to Sunken Temple trigger if the person who conquers doesn't choose to pay 1 and draw 1?

No — if the conqueror declines the "may," there's nothing to react to. You only get a reaction window if they choose to pay and draw.

Sunken Temple's "you may pay [1] to draw 1" has the "you may" as the first part of its effect, so the controller's decision is made during finalization, before the trigger becomes a Finalized Chain Item (Rules 383.3.a, 402.1). If they decline, the trigger is removed from the chain and considered to have not triggered (Rule 383.3.a.2) — it never hits the chain, so no player gets a chance to play Reactions.

If they do choose to pay and draw, the ability finalizes to the chain and then opens a normal Reaction window (Rule 406.4). This is the key difference from something like Zaun Warrens, whose draw trigger is mandatory and always gives opponents a window to react.