after go through A's story, i understand that there are alot dilemmas in btw...
A doesn't wans B to have any kinda negative thought of her,so she choose to lie...it is start from a good point where A actually wants their relationship appear in a better form(i mean a relationship without any black point,who knows the guys would think have had sex before is kinda betrayed?)
n now A worry about the lie will be revealed due to EX...

if i were A, i would choose to tell the truth, but of coz the timing and all the script hav to be prepared in advanced...im sure B will undestand one...