Is it possible for both of us get married in December if I only have a tourist visa?

You can get married but you will not be able to derive any immediate immigration benefits from your spouse if he is not yet a green card holder. Even when he gets his green card, he would have to petition for you and the process will take about 1-2 years during which time you cannot stay in the U.S. unless you have a nonimmigrant status that allows you to stay all that time.

You should not put the cart before the horse. From your question, your intended African spouse is not a lawful permanent resident and whether he will or will not become a green card holder is at this time speculative. Only after he gets a green card can he file any petition for you and then, only after you marry. Finally, ensure that you contract your marriage in a jurisdiction where same sex marriage is valid and recognized.

You can marry your fiance in any state or country that recognizes gay marriages regardless of your immigration status. Your future spouse may then petition for you on form I-130.

The law for marriages vary from state to state. It would depend on that state. Most states, from what I've heard, allow tourists to get married. And, as you know, a handful of states allow gay marriage. Of course, marrying a green card holder is no automatic way to stay here. There is a quota for visas in that category.