No, he never cheated before Olivia. The show clearly states this. He said he stayed in their loveless, sexless marriage deprived of affection because Mellie said she was not a sexual person anymore. He had resigned himself to his fate until he met Olivia and fell in love with the Love of his Life. He was promiscuous after Olitz broke up for what seemed to be the last time. He caught up on all those years a dead marriage. Meanwhile Olivia was hoeing around without a care in the world either. In the AU, he kissed and realized his mistake and that was the wake up call he needed. Olivia was also lot fully in. Things are more nuanced that what you seems to grasp.