Ah, admittedly, I left that intentionally vague, but I'll tell you, since you asked, that Harry didn't come to Severus inexperienced. In my mind, Geoff is a former lover (one of several) with whom Harry parted in a friendly fashion, because none of his relationships were necessarily serious. I imagine he had some fun, even after he found Severus. And Harry wasn't lying: he really hadn't meant to fall for Severus. It was wholly unexpected and, I imagine, he probably freaked out a little bit to realize he was actually attracted to the man. Until he got used to the idea and finally took the plunge.
LOL So, probably way more information than you were looking for. Suddenly, I feel like JKR, filling in holes best left unplugged.
I'm really pleased that you liked the story enough to wonder. Thank you so much!