Shower Shame: What is the etiquette when it comes to using someone else's shower?

“If you’re a houseguest, the shower stall and its accoutrements are yours to use... but stay out of the medicine cabinet.”
January 24, 2013

(Would you let this man use your shower?)

When you find yourself in a new acquaintances’ apartment, each area can quickly become imbued with multiple layers of meaning. Whether you walk into the kitchen, sit on the couch, or make it into the bedroom (good for you!), the physical spaces you move through in some ways represent the different metaphorical spaces your relationship with the apartment’s owner is entering.
While getting comfortable in an armchair or grabbing a Diet Coke from the fridge can probably be thought of as Category 1 or 2 intimacy, stripping naked, starting up the shower and guesstimating which one of the used towels hanging on the back of the door is the cleanest is probably a Category 5. Even though Abigail Van Buren is no longer with us, it’s pretty obvious that using someone’s shower generally isn’t acceptable dinner party behavior.
But when is it OK to pop open a bottle of someone else’s shampoo?
I had established a solid couch, entry-level fridge relationship with a friend’s new boyfriend when I stopped by a couple weeks ago for a couple of drinks with them before going out. But after someone decided to break out a board game, the situation quickly escalated. 
“You should just stay here,” my friend slurred to me five-or-so hours later after we’d lost most of the weapons in Clue. (When you’re only moving a lead pipe between rooms, it turns out the game becomes a lot less fun and more like a very elaborate, plumbing-themed challenge.)
“Ok,” I agreed, and turned over.
The next morning, I woke up about 45-minutes before work. My friend and his new boyfriend were asleep in his bedroom. Quietly, I tip toed through the unfamiliar apartment to the bathroom and, full of moral uncertainty, stared at the shower. I didn’t want to go straight to the office, but I also couldn’t exactly barge into the bedroom and ask which loofa to use. Unexpectedly, I was presented with another plumbing-themed challenge.
Eventually, I took a shower and left, but later decided to get some outside opinions on the matter.
“If a friend or someone offers you a place to say, I think it can be assumed you can use their shower in the morning,” argued 24-year-old Bushwick resident and frequent couch crasher Chris Reynolds. “I think anything in a bottle is fair game. If it’s a one-night stand though you just got to get out.” 
“If you’re a houseguest, the shower stall and its accoutrements are yours to use,” Steven Petrow, author of Steven Petrow’s Complete Gay & Lesbian Manners told me. “But stay out of the medicine cabinet. If you've had sex you're entitled to clean yourself up—but no need to go full Hollywood and use all his beauty products.” 
The day after my secret shame shower, my friend texted me about how close him and his boyfriend had been getting in such a short time. 
I told him to try the apricot scrub.