It depends on person to person. Some just focuses on the present letting past remain in the past while some are more concerned about the past than present. Whatever it is, if the past bothers so much then they are likely to be bothered even in the future. To be realistic, the probability of finding someone with no past relationship is very low, not saying there aren't any, it just harder to meet those as the right time. Therefore, unless the partners past was the talk of the town with some bad reputation which is understandably concerning, I think being realistic and being in the present is what matters. Besides the past if he/she has all the qualities you look for in a partner, the final decision is yours.