Leopard slugs (Limax maximus) mating! While dangling on a thick line of mucus, both slugs extend their male reproductive organs from their heads and twine them together to exchange sperm.
Source. And diagrams of those dreamy sex organs.
Also, because the description doesn’t specify this: slugs and most other land species of gastropods are hermaphrodites! They have both male and female reproductive organs. These two slugs are neither male nor female. They’re both. They need to mate to reproduce [because reproducing without mating is parthenogenesis and something else completely different from hermaphroditism] but they can mate with any other individual because they all have the necessary parts.
And as a result I don’t think they can be classified as straight or gay in the human sense, and I’m seeing lots of comments in the notes about how these are “GAY SLUGS AND NO-ONE THROWS A FIT OVER THESE”, as if there’s an option for a male slug to mate with a female slug. Nope. Not in this case.