One stay in the rain won't ruin it, but doing it repeatedly will. Try not to let it happen too often.
If the bike gets wet, wipe it down ASAP. Pick it up and listen for sloshing. If there is some, remove the seatpost and let it drain out and dry. Afterward spray a little WD-40 in the seat tube.
Uncoated steel rusts. What parts are steel? If a magnet clings to it, it's steel.
Keep steel painted, waxed, or lubricated with grease or oil. These steps will keep your bike in great shape to take you on more great adventures.