As long as it is done on a personal basis without requiring anyone else to take part.

First, does that mean any thought, any activity not of god, even breathing, invites these spirits in? Second, a stray thought occurred. If these are the last days, then why are fundamentalists still having kids?

Typical fundamentalist paranoia -- particularly, but not exclusively, amongst charismatics. I used to know people who seemed way more impressed with Satan and evil spirits than with their god, ironically. But it's also an effective way to keep people in line behaviorally, and keep them away from "strange ideas". Make them live in fear of "inviting in bad spirits".