Micro-cheating is the gray-area behavior — flirty comments, hidden DMs, an ex kept on speed dial — that isn't full-blown cheating but isn't nothing either.
The line is secrecy, not the act itself.
Liking a photo isn't the issue. Hiding it, or feeling a flash of guilt before you do it, usually is.
It's a conversation, not a verdict.
Different couples draw this line in different places. The real problem is never talking about where yours is.
Have the honest conversation.
Practice saying the hard thing out loud, with someone new, before it matters more.