Some common themes are Snape's complex relationship with Harry Potter. It could explore how Snape was secretly protecting Harry all along. Another theme is his past, like his relationship with Lily Evans, which is often depicted with a lot of emotional depth. Also, his role as a double - agent during the wizarding war is a popular theme to explore in fanfiction.
I'm not sure specifically as 'lewd thoughts' in fanfiction often goes against ethical and appropriate content. Fanfiction about Snape usually focuses on his character in the Harry Potter universe, his relationships like his complex connection with Harry, his past, and his role at Hogwarts.
A third theme could be Snape in alternate universes. For instance, a fanfiction could imagine what would have happened if Snape had joined the Order of the Phoenix earlier or if he had been sorted into a different house. This could explore how his character would have developed differently and how it would have affected the overall story of the wizarding world.
One common theme could be friendship. The 'pregnancy' situation might force Harry and Snape to put aside their differences and form an unexpected friendship. They could learn about each other's pasts and grow to respect one another. Another theme might be redemption. Snape, who has a complex past, could find redemption through his interactions with Harry during this strange event.
One appropriate response could be to firmly and clearly call out the behavior as unacceptable. For example, another character in the fanfiction could say 'Sirius, those comments are inappropriate and disrespectful, please refrain from such talk.'
They could also use it as a plot device for character growth. Have Sirius realize on his own that his lewd comments are wrong after seeing the negative reactions of those around him. For example, he might notice a female character looking disgusted and then have an internal moment of self - reflection.
In Severus Snape fanfiction, his past and how it shaped him is a common theme. This includes his childhood, his time at Hogwarts as a student, and his early encounters with the Death Eaters. His redemption arc is also frequently explored, as readers are interested in how he managed to turn his life around and do good in the end despite his earlier actions. And of course, his relationships with other characters like Dumbledore, Harry Potter, and the other Hogwarts professors are often the focus of these stories.
In Snape is married fanfiction, a theme that often pops up is the exploration of Snape's past and how it affects his marriage. His past traumas, his relationship with Lily, all these things can have an impact on his current relationship. The fanfics might show how he overcomes or is still haunted by his past while being in a committed relationship.
Some appropriate themes could be Snape mentoring Harry in potions in a more positive way. For example, Snape could finally recognize Harry's potential and help him master difficult potions, leading to a new understanding between them.
One common theme could be mentorship. Snape might be seen as trying to guide Harry in a strict way, and the 'spanks' could be a very extreme form of his tough - love approach to teaching Harry about the wizarding world and its rules.
One common theme is the exploration of Snape's heritage. If he's related to Salazar, it means he has a very powerful and ancient lineage. This often leads to themes of power and how Snape might use or be influenced by this power.