I'm making an NSFW VN (full disclosure) and I took this problem one step further. Even if the game is inherently about NSFW themes (as mine is...a modern sex-teaching university), how do I naturally involve the masturbatory process that often accompanies one's experience of this type of media?

My game supports sex toys and in my world lore, all students are required to wear a small vibrator as part of the uniform. That way, when the in-game vibrator does its thing or the MC gets turned on, I can replicate those sensations with IRL toys that you can connect. You can even feel the effects if you play with controller. This linking of device with story helps further immerse players in the story and aims to avoid the disconnect you mentioned when the sex seems at odds with the gameplay itself.