I think there's a big difference between portraying sexual tension and portraying actual sex. In fact, they are opposites! Instead of adding more sex and sexual encounters to the adventure, I think it's better to examine existing relationships and add sexual tension.
Think of it this way:
nobody in Barovia is getting laid and they're all wound up about it and it makes them do
cR
aZy things.
...Except Strahd, who gets as much as he wants and slyly brags about it and so everybody hates him.
...And the Vistani, who are not so repressed and so everybody distrusts them too. Except maybe some of the crazier Vistani; I like to think Arrigal and Madam Eva both have huge crushes on Strahd.
...And maybe the werewolves, who are running around behaving like animals, so everybody hates them too. Good thing we have some stern, dour wolf-hunters to chase them down and stab them repeatedly.
...And I'm not sure about the druids. I like to think of them as "married" to the absentee Strahd, and they're a bit wound up about it too, so they erect giant wooden edifices up on their hill.
Examples of relationships that can display sexual tension:
- Lady Wachter and the Burgomeister of Vallaki are both "married," but boy do those two need to get a room.
- Poor Doru in the chapel basement. His distraught father just wants Doru to keep his hands clean of any wrongdoing.
- Van Richten had to send Esmerelda away in order to keep her safe from his curse.
- You can't tell me that the Abbot is going to all that trouble for Strahd just because he thinks it's going to help people.
- Blinsky's a creepy loner and he's so much happier now that he has a monkey.
Does any of that imagery disturb you? That's kind of the point. Monsters eating people or hacking them to bits or grinding them into pies or whatever no longer disturbs us because violence is commonplace. And ordinary sex is not disturbing to us either (the way it might have been to Victorians reading Dracula). For a more "gothic" horror experience, probe the boundaries, the ones people don't even like to think about.