Navigating the Grey Area of Non-Monogamy

I’ve been in an open relationship with Sarah (not her real name) for about a year now. We’re both in our late twenties and have a solid foundation of trust and mutual respect. When we decided to go non-monogamous, it felt like liberation at first. I thought I could compartmentalize my feelings perfectly, but as the months went on, things started to feel different. Recently, Sarah introduced me to her new fling, Mark. He’s a bit younger than us and has this charming playfulness that catches people off guard. Meeting him felt like stepping into a whirlwind of energy, and there was an undeniable chemistry between us from the start. I’ve been seeing him for just over a month now, but every time we meet, it feels more intense. The problem is, while Sarah seems happy with Mark too, she’s hinted that she wants to keep things purely physical and light-hearted. She says she doesn’t want any drama or emotional entanglements right now. But here I am, finding myself getting attached, and not just physically but emotionally as well. This isn’t how I planned it; I was supposed to be okay with playing the field while Sarah enjoyed her flings. Now, I find myself wanting more from Mark than a casual arrangement. It’s confusing because I deeply care for Sarah and our relationship feels sacred to me. At the same time, there’s this pull towards Mark that is undeniable and hard to ignore. It feels like stepping into uncharted territory where loyalty and desire are at war. I’m here now seeking advice from those who might have navigated similar paths in non-monogamous relationships. Am I being a jerk by wanting more from my partner's fling? Is it normal for emotions to start creeping in during casual setups, or am I overthinking this entirely? How do you handle these grey areas without jeopardizing everyone’s comfort and trust?