It is not a problem for me, I am sure there are plenty of challenges to overcome, and adult content websites are among the most visited websites every day so user experience wise it would be a great experience.
It made sense why there was no whiteboard design challenge because the company does not follow the design process at all. As a UX/UI Designer you will receive a pre-prepared prompt (which you will not contribute to) and you will be expected to handoff a pixel-perfect high fidelity mockup.
It is fine for me. I guess there are plenty of challenges to overcome as adult websites are among the most visited everyday. Users must be very different, and from so many countries that I think it would be an amazing experience to grow as a Designer.