Just found out about this game and it sounded legit interesting... until I learned what you did, and I agree 100%. Frankly, the fact that potential paying customers are being locked out of content because they didn't pay in time is flat-out disrespectful towards paying customers.
That the developer is ignoring requests with "the game is still fun without it" does nothing to change the fact that regular players are being forced to buy an objectively inferior version if they want to experience it at all, and again comes off as disrespectful and apathetic towards the desires of potential customers. Releasing the 18+ scenes only to Kickstarter backers isn't rewarding them so much as it is punishing everyone else.
If the developer doesn't want to make Kickstarter backers mad by releasing content they paid extra for, they could always sell the patch at an increased price to what the backers paid, ensuring that the backers still get a benefit (they don't have to pay as much) while still allowing new players to experience all the game's content.
But until that happens, I'll be taking my money to games that don't feel the need to hide content from their players.