Meet Siphopteron species 1. It's a species of sea slug, recently discovered by Rolanda Lange. To procreate, two members of the species get together, stick organs into each other's heads, and fertilize. That's right — nature has at last provided us with a perfect example of the mindfuck.
I think "skullfucking" might be another word that could be used here, if only these weren't invertebrates. Still, they are hardly the only creatures with odd mating habits.