Grandma Jones, good call!
In order to commit this sin you would have to know exactly what it entails and then willfully do it with malice.
I can tell you that the Gnostic interpretation of this sin has everything to do with abusing sexual energy, which is the most powerful energy that a human being can generate...but I'm not going to go there in this post. Research it for yourself if you find that plausible.
Since there is no "eternal destruction" like the Watchtower teaches, there really is no sin that is totally unforgivable, in the sense that your soul would be eternally destroyed. Above this dimension, the physical plane, time and space is collapsed. It is not linear, everything is quantum. But egos are dissolved in the realms below, the mineral, plant and animal kingdoms. What that means is that you would have to pay the karmic debt for that sin, and that could take a VERY LONG TIME. Even the WORST of the WORST of evil people will eventually turn toward the light, because that is the destiny of the collective soul of man, and along the way all their egos will have to be dissolved. In other words, they may have to go through the whole trip around the Wheel of Samsara, which could take 25,000 years. Still, nothing is "eternal" except God Consciousness.