I would say trust. Especially if you have never had even one faithful partner. Or if you just have difficulty trusting. I’m in that boat. I’ve never had a single partner who was faithful. Now, even though I love @Rain Dew with all my heart, I still don’t trust her. I’m trying, but my paranoia and expectation that she will cheat, prevents me from trusting her. As for everyone else, I’m not willing. I refuse to trust anyone. It’s not a “can’t”. It’s a “won’t”. It a conscious decision not to. No different that choosing to wear a black shirt.
Trust is extremely important, but also, extremely difficult.