Did this while staying at a hostel (One80 I think) in Germany. I traveled with a small quick drying towel so it didn't take up much space, but it barely wrapped around my waist, so I would just throw it over my shoulder after a shower. There was a cute German guy in the shower too, looked like he was 18, and spoke barely any English. He was shaving at the sink in a towel, and I came up to the one next to him naked. I began shaving, but slowly started to get hard. I could see him looking out of the corner of his eyes, probably since I was circumcised and he wasn't, and both of my hands were busy with shaving, so I didn't really know how to cover up. I also didn't want to touch the sink ceramic, so I was backing up and bending over slightly to conceal my boner. I was completely hard by the time I was done shaving.