Recent studies have found that in the US and Europe, marriage is associated with stable economic conditions, while separation and childbearing within cohabitation are associated with disadvantage. Few studies have examined relationship quality in shaping family transitions, especially analysing interactions with socioeconomic status, which could help to explain the divergence in family behaviour. Using the UK Household Longitudinal Survey (2009-2017), we employ competing risk hazard models to follow respondents as they 1) transition from cohabitation into marriage, childbearing, or separation; 2) transition from marriage or cohabitation into parenthood; and 3) separate after having children. We find that the happiest couples have much higher marriage risks, but relationship quality is not directly associated with childbearing in the UK. Instead, the effect of relationship quality on childbearing operates through marriage: the happiest couples marry, and married couples have children. While low income, low education, and partners’ unemployment are associated with childbearing in cohabitation and separation, these associations do not differ by relationship happiness. Thus, our findings suggest a “relationship quality bar” for marriage and separation, but not childbearing.