I just came back from a sightseeing holiday with my sister. We returned to the hotel mid afternoon each day, just early enough to do a bit of sunbathing. So I stripped off on her balcony each day for five days. Then on the last day it rained. She asked me to strip again, so I did so she could take some photos. Although she had seen me naked all week it turned me on, being asked to strip and being told how to pose.
I guess this almost counts...
In halls at uni, i'd had a shower and walked back to my room with just a towel around me. The person in the room opposite me had some of her friends round for a bit of pre drinking, one of her friends saw me go into my room so she decided to follow me in. I hadn't taken my towel off when she walked in but as soon as she shut the door behind her she started pulling at my towel. Deciding to make it a bit more playful I gently resisted until I just let the towel fall to the floor.
I was rock hard by now and probably in part to the alcohol in blood stream she decided to sit down on my bed and told me she wanted to watch me play with myself. Of course being the gentleman I am I obliged and started masturbating in front of her. Several minutes later whilst we're still having a great time my flat mate bangs on my door telling her friend that they were now going. She decided that she'd get the taxi with her friends but told me to meet her out later that night. So I agreed to that (who wouldn't!?) and when she opened the door to leave everybody looked in and saw me standing me rock hard!
Needless to say I went out that night and ended up back at hers and we finished what we'd started earlier!