Step-Parent Thing

So, I'm married with kids and we've been together for a while. One thing that pops into my head sometimes is this weird fantasy about having an affair or something like that with one of the older step-kids from my ex's previous marriage. It's really just a thought that comes and goes, never acted on it, nothing real happened at all. But I do find myself occasionally wondering what would be different if I allowed something like that to happen, you know? I mean, they're grown up now, but there's still this weird feeling of power dynamic when an adult is close with a kid from a previous relationship. It's just this idea though, never anything real or serious. Just one of those things people don't talk about much.