"These are adult men we are talking about, surely even the most naive would not accept this sort of thing as being all part of the gay scene. "
There's perhaps the point that younger men might have little or no experience. And if that happens there's always the possibility of being caught like a rabbit in the headlights. If you have no experience, you might not know what to expect, and then things can easily happen that, with more experience, you would understand either as assault or as something you didn't want to happen really. But without this experience it's hard to say "no", or to recognise what's really happening.