Very interesting question. If the girl did it while you are already together, we call this "cheating"... If it is the past, then who has no past... If any has not sinned, let him cast the first stone.

we all have a past that we try to escape. is she is trying to escape her past or embrace it? talk to her about her infidelity and admit that you know and are willing to work for the sake of love. if she is open to the idea give her time to come to an overstanding of what real love by watching her man lead by example. it won't be easy, but with patience, communication, endurance, prayer and faith all things are possible. as for you , place abstaining on your to do list. in the absence of sex take time to find out who your lady is spiritually. pursue the inner woman and as your love grows or dissipates you will be in a position to handle either outcome. remember repentance is the key; ask her quesyions like is there anything i can do better for you or do you need more of me in any way. reach past the burning questions like why and other incriminating things that will hinder your cause. Jesus died so that thru Him we will be forgiven for all sins. so when the heat of the trials come, ask what would Jesus do?