IMO, there is a difference from wanting to keep something private because trust has not been established, and saying untruths to protect an opportunity to deceive a person about the true nature of your relationship with them or others.
Is the lie you are talking about from the situation in your other thread - where a woman you've been seeing got a ride to meet you from her ex, when she had clearly told you she was getting a ride from her girl friend?
These are what many people call the "gray areas" of dating... when you are not yet monogomous, but you are watching for indications of trustworthiness. It is a critical time.
In early dating we make judgements about , as WaterTiger says, someones "worthiness". Is he/she worthy of my trust? Am I safe to open up to this person and give them my love?
When people lie to us, they rob us of the opportunity to be truly free in our ability to love them - because they have undermined our ability to make an informed decision regarding our hearts. When the lie is exposed we feel all our judgements from that point of the llie on are at risk of being made on deceptions our hearts and minds thought were truths. While we may be able to intellectualize all the reasons why the lie occurred, the emotional damage is still very real.
Lies undermine the safety and security in a relationship that is essential to trust. When trust is damaged, of course the potential for movement into a more meaningful relationship is undermined - because trust and intimacy go hand in hand.
When someone offers up their justifications for lieing - and they are fear based as Interceptor has said - usually lies are told out of the fear of them not getting what they want.
It is an indication that their want is more important to them than honoring your need to be able to be informed so you can take care of your emotional safety. I say need, because I believe all people, male and female, need to feel safe in the giving and receiving of genuine love.
Just because a person admits to you that they lied, and offers up their fearbased explanation, does not suddenly mean they are now funtioning from a core operating system of honestyand integrity in their dealings with you.
It usually means they have done a recent loss assessment calculation and decided it is then more beneficial for them to be honest now as the honesty serves them better than the lie did before, so then they "fess up" - but it is still all about them having their needs met.
This confession/explanation of the "why" of the lie is rarely a new loving, honoring, respectful and conscious provisioning of your emotional/relational safety - but rather still comes from a place of them getting their wants and needs met - it is just that the truth now serves that purpose for them better than the lie did earlier.
Note that their neediness is so strong that they are willing to put your emotional safety at risk - as well as the overall level of trust and intimacy of your relationship with them, at risk as well. It is a sad thing, truly.
Now, we can forgive. The ability to forgive is a sign of spiritual and emotional maturity, but forgiveness does not automatically mean we should contine in a relationship with someone. We can feel compassion for their functioning out of fear, we may even feel pity for that exposure of their humanity, but it does not automatically follow that just because we forgive someone and feel compassion for their fear, we automatically trust them as if the lies did not occur. It does not mean that the heart simply opens itself up to loving them even more.
Lies and deceptions - particularly ones that introduce a third party into a romantic relationship dynamic, do not aid in building romantic attraction - nor in the critical path of surrender that is required by both men and women when love of a great depth is to be shared and experienced.
Love is intimately tied to respect, trust, freedom, safety, and security. Lies undermine love in everyway.
Liars will have us think that love is unconditional - but romantic love is conditional. It is the unique nature of authentic romantic love that conditions are to be met - or we would just go around falling in love and giving our hearts away frivolously/indiscriminately to any and everyone, without any recognition of love's value - or the value of what is truly shared between a man and a woman when they love well.
Often adults will carry an unconditional love paradigm of Mommy or Daddy over into their adult relationships and then fault their partner for not loving them unconditionally, as a Mommy or Daddy (idealistically) would. So then the liar often switches the responsibilty of the damage to the relationship on to the shoulders of the partner not loving them enough and not accepting them unconditionally - and thereby they bypass taking responsibility for how their lieing undermined the potential, and/or actual, stability of the relationship. A Parent-Child love dynamic is not the same as a healthy adult Man-Woman dynamic.
It is really important to be awake and aware, so we are making informed decisions - and empowering the people around us with the truth, so they can make informed decisions as well. The fact is people do get hurt in love, people do disappoint us, we disappoint them. Being mindful of motivations is important - our own as well as others.
Everyone has a dark side, everyone has a past that has choices in it they would not have made if they knew then what they know now.
It is important to be careful that any "forgiving" that takes place is not actually a silent agreement that you are then both allowed to deceive in the future. In other words, if you forgive her for lieing to you now, she owes you a forgiveness of you lieing to her later. Relationships built on silent agreements are not built on accountability and are not conducive to authentic loving, but rather are designed to serve comfort levels of stagnation, rather than genuine growth.
Instead of sharing our fears, our faults, our weaknesses, our failures, our needs - we sometimes lie. The courage it takes to say "I need..." to another person makes us vulnerable to rejection, ridicule and yes - vulnerabe to having intimacy occur - which can also be scary. Yet if we pretend the need does not exist - or we do not bring it to our partners, then there is no chance of having a thriving relationship.
It is important that we not give more to a relationship, vulnerability wise, than is wise - and it is also important to note that exposing our need and desire for intimacy with someoone is a valid need - now whether or not that person is capable of meeting us at that level of trust, trustworthiness, and intimacy is something that needs to be defined on an individual basis. It is up to each of us to decide those boundaries for ourselves.