from a gameplay perspective if it's coherent with the setting, like "peasant quest" for example, medieval fantasy on rpg maker engine, where everything fucks everything, it would make sense to have stds, and it's relatively easy to implement, it's just a switch (the sickness exist in the game and you have it or not).
the thing is, what does it add as gameplay value for the player ? if you punish the player for having sex in a porn game, you have to make it a fun way to deal with, with manageable consequences (implying it not to be a game-over condition)
maybe it IS the purpose of the game and you have to spread it ? could be a fun idea to play around with as well.
Like always in porn it's just a matter of fetish, you can't force it on peoples that don't wan't it (like adding it midway through developpement, such as forcing out of nowhere a ntr scene in a romance dating sim) and if the game has it, it must be stated by the developper from the start as his intention on the planned fetishes.
Make me wish for a w40k parody game about a plague bearer of Nurgle... :x