Sex Content can be considered a
Value-Add on top of an existing game.
Let's say a player likes the gameplay of Slay the Spire.
The problem with that is you are competing with Slay the Spire and the many clones of it.
You can't be mediocre, you can't be average and just good enough, you need to surpass each and every one including Slay the Spire and it's production value and polish.
But if you add Sex Content the equation changes, the Value of Sex Content is in
Novelty, new characters, new scenes, new fetishes/kinks.
Since Sex Content is
Consumed since it is only novel Once then it is ever in Demand.
So what constitutes "good game" can be a average game that doesn't have to do anything that special since it's already something they like. Instead of repeatedly replaying Slay the Spire they can try your new game that is good enough to satisfy that. What they get in addition is the Value from the Sex Content, so as much as you add through expansions and sequels. Of course if you are going to make expansions it would be best to add value to the gameplay also.
Goblin Burrow is the most shameless example of this, they basically re-released the same game 3 times including new DLC expansions for each game with very minor changes to the gameplay.
Still the most consistently best selling game on DLsite.