Jon,
The rugby song repertoire by its very nature is bound to have more lurid scatological and sexist lyrics than any other genre. Even the forces songs of WWII contain a wide variety of material with SOME lurid pieces and you would expect a similar smaller percentage of more extreme material among seamen.

Here are a few possible reasons: The rugby groups revel in the extremeness of the material and they are ALL young men delighting in each other's company and competing to be as bawdy as possible. Seamen and forces personnel and indeed comparable groups have a wider age-range and spread of backgrounds, so you get a much wider range of material, sentimentality, subtlety, cleverness, wider range of sources etc.