The key component to an open healthy relationship is creating safety around honest communication.
By creating a safe environment and protocol for personal expression and discussion, it becomes possible to talk about anything with confidence that your partner will listen to you, and focus on how to move forward together, rather than sitting in judgment, or focusing on placing blame, "changing the rules", or the like.
Every relationship is based on mutual understandings forged at the outset. And every relationship changes over time. It is vital for success, then, that every relationship "hold space" for open discussion without fear of punishment or retribution.
This isn't the same as saying that any transgressions in a relationship (cheating on a spouse, say) need to be forgiven, merely that discussing attraction to someone outside the relationship before any cheating occurs should be something that has its place – to encourage a re-negotiation or reconfirmation of boundaries, at the very least.
This kind of support process doesn't happen overnight, and even long term relationships can suffer from a "crusting over" of assumptions, or a weariness to "have to talk about every little thing that comes up". Nevertheless, problems and assumptions that aren't addressed when they're still new/small, will just keep growing, until they become impossible to solve, even when you both agree you want to solve them. It's a lot like going to the gym – start off slow, gauge your limits, and then make it a regular part of your routine until you actually like "going there".