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.
Be open and inclusive with the client about your process from the beginning. Gather as much evidence and data to show the client why you arrived at your solution/rationale. And finally, stick to your guns!
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.
I basically walked through some of my projects sharing my screen and answering specific questions about my design process. Also during the onsite interview, I had to present some projects to the design team and answer some questions about it.