A Slippery Slope

I've been in an open relationship for a while now, and it's mostly about experimenting with different connections rather than serious commitment. But recently, I found myself getting really intimate with someone outside of our arrangement, and the more we did, the harder it became to justify it as just experimentation. It feels like I'm sneaking around instead of being open and honest, which is completely opposite from what I stand for. How do others handle this kind of emotional entanglement in an otherwise flexible relationship? Am I overthinking this or should I be more straightforward with everyone involved?