With a traditional video game, I can sometimes put up with bad graphics (purposely or otherwise) if the gameplay is really fun or the story engaging.
It's harder with porn games as the visuals help a lot with why you're likely playing the game in the first place, as wank material. I've tried to get into a few text-based games or games with limited/bad art that people swear by, but If I'm not playing the game for the porn, I would rather just be using that time to play a legit video game, of which I have many in my backlog.
Graphics and the plot are there to get me immersed, though a plot isn't always needed for me. Good graphics with bad gameplay, however, is where it gets iffy, and whether or not I drop the game depends entirely on my willpower.
It's two ends. A game with amazing plot and shit graphics can be good. A game with shit plot but amazing graphics can be good. If you have okay both it's usually worse. I'd probably prefer to "play" something with a better plot.
Depending on the situation, both are pretty important factors for me to start and continue playing, but if I absolutely had to choose one over the other, I think I’d go with the graphics.
Pretty girls are important, broooooo!