There isn't a limit, your tastes on what you consider "attractive" will change over time. When one is young one only thinks on reproduction, and the older one gets, the more prevalent just having someone who you enjoy keeping you company.
Speaking in terms of ultimate (evolutionary) causes, obviously, but on a proximate level of, for example, what the people in question actually think and feel, as well as the practical outcome given the accessibility of effective contraceptives, it has nearly nothing to do with reproduction.