Cuba has been slowly increasing it’s legal protections for same-sex relationships and LGBT people since the late 70s and is expected to legalize same-sex marriage through the Family Code after pressure from some Christian groups prevented the National Assembly from adding it to a new draft of the constitution in 2018. If you’re asking about how a hypothetical new party in a hypothetical new proletarian state would legalize same-sex marriage, it would be important to ask why homophobia in a party would be so widespread to begin with. A party committed to democratic centralism would have to allow it’s policies and it’s standards for evaluating them to be questioned. People would have to be allowed to challenge current policies and discuss the material impact those policies and proposed changed would have on society. People today have access to enough information to easily make and defend the argument that equal rights for same-sex couples would have a positive impact on society.