It's simply a matter of hormone release. Thinking about sex and sexually related topics, especially over a long duration - like writing a story (writing for clips of hours at a time), causes the release of hormones that stimulate sexual arousal, which is far more obvious in men because they'll start producing pre-ejaculate while writing, which is both a curse and a blessing. Women will also produce fluid preparing their vagina to accept anticipated penetration. These are completely natural reactions that are unavoidable. Those hormones and the reactions they cause are what sexually arouses you. Therefore, if you're writing a sexually related story that isn't making you horny, it's not good. It's dispassionate. If you're not passionate about what you're writing, it shows, and it certainly isn't the best you can do.
Drawing isn't as involved with step-by-step development of sexual scenes and is mostly a mechanical process, so there's big breaks that interrupt hormone release because you're thinking more about the process than the actual topic. You're not constantly playing out an entire story in your head, visualizing each and every move, emotion, consequences, etc. Writing is much, much more simulating.