Stephenie, I have to remind you that this couple has been a couple for just over 3 months. Even with 24/7 discussion, there are many things that have not been brought to the forefront yet. To call this a "lie" is just not right.
I am sure that the OP has many details in her life that she has not yet mentioned, even in passing. Many that could be deal breakers for her B/F if he knew.
We all attach different values of importance to the things in our lives. We all have stages that we go through, and times when it is "right" to divulge.
Perhaps this relationship has not yet reached the point where this disclosure was ready to come out.
Only with the fictional "Vulcan Mind Meld" could a couple share everything in an extremely short time.