Spike wasn't so much 'already naked' as 'still naked,' since he slept that way and hadn't bothered to put clothes on when he woke up. Unlike Riley, though, he didn't feel the need to cover any part of himself when he went upstairs. Generally speaking, his need for comfort -- and hiz laziness -- outweighed whatever very tiny need for modesty he might have had. "There are other rods I'd rather be holding... oh, I don't feel good about that one. Way too easy. Give me a challenge next time." Not that it stopped him from slipping a hand around Riley's cock anyway. Obvious or not, any excuse to touch Riley was one he'd take.