Final answer:
The provided information does not adequately answer the question regarding the percentage of relationships with heterosexual men as primary breadwinners. However, there is evidence of changing family structures with an increase in cohabitation and the acknowledgment of same-sex couples.
Step-by-step explanation:
The question regarding what percentage of relationships have heterosexual men as the primary breadwinners does not directly relate to any statistical findings provided in the references given. Therefore, it is not possible to give an accurate answer based on the provided information. Historical shifts in family structure and roles, such as the increase in cohabitation and the presence of same-sex couple households, show that the concept of primary breadwinners is evolving. We know from the U.S. Census Bureau that unwed cohabitating opposite-sex couples constitute 10% of all opposite-sex couples, which highlights a change in traditional family roles but does not directly answer the question about the percentage in which heterosexual men are the primary breadwinners.