Yes, both Pittsburgh and Philadelphia offer employee benefits to domestic partners. One reason is that legally, marriage is a state thing not a city thing. City's can choose to go beyond the state in certain areas, such as the benefits they provide to their employees, but they cannot create a marriage. The benefits that these city provides are few, numbering less than 10. There are over 600 state laws and 1200 federal laws affected by marriage.
Many heterosexuals oppose marriage but favor domestic partnerships.
Separate but equal is not equal. The rights, responsibilities and benefits of marriage cannot easily be recreated. Many people have a confusion because, in America, marriage is two things. There is religious marriage and civil marriage. Civil marriage is the contract with the state that creates a family.