Personally, I’m with most who’ve responded already. I can’t imagine getting an accidental hard on in the steam room. The steam definitely has an effect.... not one I mind, btw... as my cock tends to get plump and hangs reeeal low. I wish it always looked as impressive soft as it does in the steamroom. But to get a hard on, I’d really have to be exercising my imagination and presumably creating some friction.
It’s not that you’re fluffing yourself up with a semi to impress others in the locker room and fighting the challenge to remain partially hard without going full hard on, and losing that battle? I’ve had friends admit to the fluffing thing... don’t think it’s uncommon... also, not necessary.
Or, just taking your post at face value, you get hard in the steam room. I say, “Right on. Who cares?” I wouldn’t make a deal of it if I saw it, and anyone else with a penis should be just as enlightened. Penises get hard sometimes for no rhyme or reason. If this is the case for you, I’d just say to rock it. Anxiety over it probably only exacerbates the issue, I’d guess. And if it really bothers you, then take the towel recommendation.