I saw your post earlier, but had to out shopping.
Although I have not owned boy dogs myself, its normal behaviour and something there bodies need to do. Any females in season will encourage the behaviour more.
Many owners of boys will be familiar with some form of "love cushion"
It wasn't so much the fact that Trixybird walked in on him pleasuring himself, it was the very stained and crumpled copy of the dog magazine he tried to push under the sofa.
Spoilsport now I'll have to guess about it from the replies, but if it said what I think then I've only ever had one unneutered male of mine do a similar thing but have also had a neutered male try though no staining.