Secret Fantasies and My Partner's Backyard Fence

I've always considered myself pretty open with my partner about our sexual desires. We're both in our twenties and married, but there's this one fantasy that I haven't shared because it feels too taboo. A few weeks ago, we went to a friend's place for a dinner party, and during the evening, something caught my eye, a narrow gap between two backyard fences behind their home. The thought hit me like lightning: the idea of standing in the shadows there, exposed but hidden by just a few boards apart, looking out at couples walking or maybe even kissing nearby as they passed by. It's this strange mix of being seen and feeling anonymous, vulnerable and yet protected from real contact. What if someone were to catch a glimpse of me? The thrill was too much; the possibility still haunts my thoughts. I've never acted on it, but imagining it sends shivers down my spine. How do I confess something like this without scaring my partner away or making them feel unsafe?