Is failing to mention the entire truth a lie?

If they specifically ask you and you don't tell them, it's a lie. If you just don't tell them and it isn't raised, it's an omission and technically not a lie.

I would say a little it is... its good you dont act on them as it would hurt the person you are with...
But its good to be open with the person you are seeing and you should let them know how you are feeling but assure them you have not acted on such urges.

If we all said what was in our minds, we'd all hate everyone.

It's a bit of a half truth, but as long as you don't act on your urges and stay true, I see no reason to share that.
No reason in saying something that I'll have to explain later.

The fact you are asking means you already know that you are being dishonest.

You are clearly being deceptive, your partner wants to know if you are still having those urges, probably because they care for your wellbeing, and you are hiding that which you know to be the truth.

You may not be directly lying, but you are lying by omission and intent.

This is so weird because I am living this exact situation with my boyfriend right now. This is what happened (short version): he told me he wanted to try open relationship, I violently opposed to it, as he knew I would. I asked him tons of questions, he barely answered. We almost broke up, but I didn't want to. We talked and he finally told me that it was a misunderstanding and that he didn't absolutely want to bang other girls, but that he needed more out of this relationship. I still hope he told me the truth and won't act on those thoughts. So you don't need to tell her if you think it will do more harm than good. If you do talk to her about it, I strongly advise that you thoroughly question yourself about what is going on. Do you really want other girls or just other sexual experiences that you could get with your girlfriend? Ex.: My boyfriend is really into bondage, but is shy to act for fear of pushing me too far. Thing is, I love that stuff too. We just never really talked about it.

No, a lie is when you twist the truth around. If you don't even tell them then it's more like a secret.

No - by definition, not mentioning the entire truth is not a lie.

It may still be deception and dishonesty, though, depending on the circumstances.

In your case, withholding the truth wouldn't necessarily be dishonest, since your partner knowing you identify as polyamorous isn't really important or beneficial to your relationship.

However, it does suggest you don't think your relationship with your partner is built on a solid enough foundation to hold up to this 'revelation'.

It's a half truth which can not be as bad as a lie, as bad as a lie, or worse than a lie.

i think if you told them that you were polyamorous from the start but still choose to be with you. it is their own dumbass fault for getting cheated on.