For someone just stepping into the more adult side of fantasy, you'll want a story where the romance and heat are woven into a plot that actually holds up. A lot of people jump straight to Sarah J. Maas, and her 'A Court of Thorns and Roses' series is a common gateway, but sometimes the fantasy politics overshadow the steam for newcomers. I'd actually steer a new reader towards 'From Blood and Ash' by Jennifer L. Armentrout. The world-building is straightforward enough to follow without getting lost, and the slow-burn tension with the protector character, Hawke, pays off in a way that feels earned. The fantasy elements—vampire-like creatures, prophecies—aren't overly complex, so you can focus on the character dynamics.
If you're looking for something that feels more like a single, contained adventure with a lot of chemistry, 'A Touch of Darkness' by Scarlett St. Clair is a fun Hades and Persephone retelling set in a modern-ish underworld. The power dynamics are clear, and the romantic scenes are very much a focal point without requiring a huge investment in lore. I found the pacing a bit rushed in places, but it never loses that sense of fantasy escapism. It's a good pick if you want the smut to be prominent but still enjoy the trappings of gods and magic. The sequel delves deeper into the mythology, so you can decide if you want to continue.