Formerly titled "Presentation and You! An informational guide."
Dead Weight
Let's say you are developing a (non-novel) porn game, following the average porn game's formula. However, halfway in development an issue relating to the porn arises, and that is "Why must there be porn?". This might seem like a dumb question, but think about it. If a game developer just added whatever for the sake of having it, their games would be a jumbled mess. The porn in this case is dead weight and not contributing to the gameplay, and for future reference dead weight-porn (haha nero). When it comes to game design, it's important to make sure that everything has something to give to the gameplay, otherwise it might end up being detrimental.
However, not everyone decides to get rid of this dead weight. Either they are simply inexperienced, stubborn, or simply don't care too much about the quality of their game. If a game succeeds like this, which it likely will since most people care more about fetishes than game quality, it will inevitbly inspire someone else to make their own game following a similar poor formula. Then another game is made, more people are inspired, and the cycle continues.
So, how do we keep the porn without damaging the quality of the game?
Presenting: Sexual Game Mechanics
This is a little straightforward, but having the porn pull it's own weight (referred to as sexual game mechanics, or an SGM) is the key to this problem. However, due to their nature, sexual game mechanics need certain conditions to thrive, since they are a more specific category of game mechanics.
The first and most important condition is the relevance of the SGMs. If there isn't a point to the sexual content or a reason to pursue it besides "It's sex", it's just dead weight. SGMs should be part of the main gameplay loop, like the fighting in a fighting game. However, it doesn't always need to be battle-fuck mechanics, since that might not work in a colony simulator. What the sexual mechanics do should make sense for the game's genre.
The second condition is pacing, where the sexual content needs to be able to keep up with the pace of the game. Because of how sexual game mechanics are sexual, the pacing for them can't be too slow or too fast. Not every genre makes a good porn game, like grand-strategy games. However, this isn't a very
restricting condition since the pacing is acceptable as long as the in-gameplay sexual content is enjoyable.
Lastly, is scale. While this one is a little self-explanatory, a game needs sufficient amount of sexual content to avoid repitition. If there isn't enough content for the frequency of the mechanic, players will quickly get tired of it.
Ultimately, SGMs are still game mechanics, and are to be treated as such. They need to be balanced out, polished, and fun, like any other (generally speaking). SGMs aren't always inherently good, just as some game mechanics can be bad. I can't guarentee that following all the conditions above will make a perfect porn game, since one or two good mechanics can't fix a bad game alone.
Reguarding Other Sexual Content
Galleries
Now galleries aren't inherently bad, but they often encourage dead weight-porn. While the idea of letting the player revisit sexual content quickly sounds good, this is often used as a crutch to avoid making (good) SGMs. This doesn't mean you can't have anything resembling a gallery, but it's a good idea to pretend it isn't there for the sake of fap-ability during gameplay.
Game Over Sex
Again, this isn't an inherently bad thing, but it is usually misused. The obvious problem with this is that the sexual content (something the player desires) requires you to play poorly, contradicting the idea of rewarding skill. This isn't an issue if it's only a bit of the sexual content, but when it's the ONLY sexual content, it starts to be problematic.