Like you're really enjoying the roleplay, the world-building, the other person's character and their personality, the adventure you're going on, even the OOC. But you know it's coming. A character is going to get horny. And you're going to have to break away from plotting and actual story telling to get their jimmies jammed, and that can be a couple pages worth of content, and by the time you're done all your energy for the parts of the rp that actually interest you is gone. And you realize the soul sucking truth. You're not bad at writing smut, you just don't like doing it because it breaks things apart frequently and there's no way around it on a site like this.