My idea for now was going to be about a threesome (a polyamorous, committed relationship of three adults) and their gothic lifestyle, their love for each other, and their love for their two children, who they raise together.

I think the point of the story would be about how really, even though society may say conflicting things, that their relationship and lifestyle is actually quite loving and healthy. I will have all kinds of things that threaten to, or do, tear them apart. There will be a lot of romance, perhaps the story IS a romance... about the power of their dark love. Inf fact, I think a lot of the emphasis would be on their love. However, it also seems like it is drama to me. It deals with societal things, and raising children, and life thigns as well. Plus, whatever the subplot is.. well, that could also be drama or something. Or maybe it would be more romance? I think I'ma little lost.

What makes a movie a drama and what makes it a romance? Can it just be equally both? How do I know which one it is, if I had to decide?

Just because something involves love/relationships, doesn't immediately classify it as a romance. If your movie is full of dramatic scenes(usually involving crying), hatred, betrayal, than its drama. Also, romance usually involves two(or in your case, three) people falling in love. It sounds like the characters in your story are already in some form of matrimony, so I wouldn't consider that falling in love! My vote would be drama.

You've got to find what the main thread of your story is: what do you want to climax on and have things turn on, the characters' finding of love or their triumphing over societal/personal/etc. issues? If it's the former, then it's a romance; if it's the latter, then it's a drama. Think about Little Miss Sunshine maybe and the relationship between the mom and dad as an example of what I'd think of as a drama that still contained some romance. (or the series Big Love if you're into tv)

I'm struggling with a similar kind of question. My story is about a character finding herself and coming of age, in a sense, which I think makes it technically a drama, but romance is a big part of it and the feel is much more romcom and light. Don't know. In any case, good luck!

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As long as we draw breath, there'll be some things worse than death.

Well yeah, the thing is, it would climax on the characters finding love and having that love triumph over societal/personal issues. It seems like both to me. I would label it pure romance but then they also realize how much they love their children, so... I wanted to show a real, dark love between the three of them, and how that love is more important to them in the long run, and how their love of their whole family (children included) is more important to them than what society thinks. But the main emphasis would be on the love if each other (the three partners) and how that triumphs over society and other issues.
Maybe some things are specifically this or that? I don't know.