Caught in the Moment with a Student’s Partner

I’ve been teaching at Lincoln High School for five years now and generally keep my personal life separate from work. Things have gotten complicated recently, though. I started casually dating Sarah, a new history teacher who just joined last month. We’re both clear that there’s no exclusivity between us yet, but we’ve become pretty close over the past few weeks. Yesterday afternoon, I received an urgent message from Sarah asking to meet her at my school right away. It turns out she had to cancel a lunch date with her boyfriend Mark, also a student here. He was understandably upset and wanted to vent his frustrations to someone he knew would understand his predicament as both a student and athlete. Mark came into the faculty lounge where I was having coffee around 3 PM, looking distressed and needing someone to talk to. As we chatted more deeply about how stressed Mark felt with balancing schoolwork and sports commitments, there was an underlying tension in the air. Sarah had mentioned that Mark often confided in me about his academic challenges in her class. I tried my best to provide support and encouragement for him, but as the conversation continued, I noticed a shift in the dynamics between us. His demeanor became more relaxed and playful as he opened up about his personal life and joked with me about how Sarah was sometimes tough on him academically. That’s when it happened, an unexpected spark ignited between Mark and me. He leaned in closer, almost unintentionally brushing against my leg under the table, causing a rush of excitement to flow through me despite myself. We sat there talking for another hour or so before Mark had to leave for practice. After he left, I was left feeling shaken and unsure what had just transpired. Later that evening, Sarah called and mentioned she’d spoken to Mark who seemed calmer but still needed more time alone with her. I didn’t mention our accidental meeting at school again until she brought it up herself during a call last night. Sarah and I discussed the situation in detail. We both agreed there was no physical crossing of boundaries, but we couldn't shake off the sense that something had happened between me and Mark while he was with Sarah. I’m writing this now because I need to know if what I felt was simply a misread moment of sympathy or something more serious. Is it normal for someone in my position to feel such intense physical attraction towards an older student's current partner? What’s the right way forward from here without causing harm?