Like others, I tend to use present in short things (especially porn) and past for long. Part of that is that longer stories are more complicated, and I find that past/past perfect/etc all work together more seamlessly in my head, where the same constructions in something that's overall present tense feel more... awkward. I suppose it's just more familiar, but I find I have to THINK a lot more about what the specific verb form should be in present tense stories where I rarely have to worry about it in past.