Re: Relationship built on lies
She is very young and very insecure. The behaviors you describe are those of someone who has very low self-esteem--promiscuity and showing a different face to each person/group, so she will be acceptable to them.
Because you are already married, you will have to make a difficult decision. On the one hand, there is tremendous benefit if the two of you start marriage counseling and she, at least, starts individual counseling as well. As she changes, she will need you to change and grow, too, or the marriage will be at risk for different reasons. But going through this and learning to grow together will give you a very solid basis for a future together. You may need individual counseling too-you rushed into marriage and were fooled by someone who was not happy with herself or able to be herself. This is not a criticism--lots of people make this mistake. But, you may have some insecurities and her attachment to you may have bolstered your poor self esteem. Consider this possibility; ignore it if it doesn't seem valid for you.
Your other choice is to cut your losses now. Counseling can make a huge difference, but it may not be enough. She could have very deep-seeded issues that will take many years to work through, and she might continue to lie for years. (Promiscuity and low self-esteem can be signs of former sexual abuse, too). You are both young and maybe moving on is best for you.
There is a child involved, however, so if you think you can give it a couple of years with lots of counseling, do it. At least then you can tell your son you tried, when he is old enough to want information. And counseling may help you divorce as a mutual decision, which can greatly decrease the animosity between you--and an amicable divorce is best for your son, if it comes to that.
Talk to your wife and let her know that her sexual past is not the issue, that the lying is. Rebuilding trust will take effort and time and lots of counseling, and is she willing to do the work to try to make a go of it? There is no shame in saying "we were just too young," either. Have a heart to heart and then judge her sincerity by her actions, not her words--and tell her that is how it has to be until the trust is rebuilt (and again, that takes years). If she commits to counseling and all that goes with it, and you begin to see other, small improvements over several months, you will know she is sincere. If after 6 months you cannot see any evidence of consistent commitment, well, you have tried.