thank you for sharing your question with the Wefugees Community!
I am sorry you are receiving your answer this late but as you probably know Wefugees is a plattform where mostly private individuals are answering questions and therefore not all questions can get answered in a short time. I am new to this plattform and still would like to answer your question, so that either you or other people in your situation maybe still receive the information they are looking for.
Regarding the procedure and the prerequisites for a marriage in Germany I found this very helpul article that lists all the important information. There should´nt be a difference for same sex marriage: https://www.internations.org/germany-expats/guide/29461-family-children-education/marriage-in-germany-15962
It also explains in there that for the procedure of getting married itself you can get a tourist visa to come to Germany. Also note that after being married you can be eligible for a resident permit if you wanna live together in Germany.
Regarding your question about the place of the marriage: normally you get married in the district where you are living in. This would be in your case the district of your boyfriend in Germany. Unfortunately I don´t know if there are any big differences regarding certain cities or districts. But maybe someone else in the Community has made similar experiences and can share those!
All the best