Ha! I just subscribed to your comic 
I'm waiting for the answers because I have the same problem.
I have a story (comic) with a gay couple and a non-binary secondary, but important character, but the story is not really heavy on romance and I don't speak a whole lot about the non-binarity of that other character. It's only a facet of the story.
I hate the idea of warning people of such themes. I have no special interest in warning homophobes, and people who are simply not super interested in lgbt themes can still enjoy my story for all the rest of it.
I don't want either to get readers who would read my story ONLY because of that aspect, because they may be disappointed when there are whole chapters with no mention of that.
But I still wanted to have some mention of the lgbt themes because there are also a few chapters mostly focused on that. I really like the idea of having these themes introduced naturally in stories that mostly speak about something else, so I think there may be other persons who would like that too, and I want them to know that's what my story is about.
I also think of people, especially young ones, who have no problems with lgbt themes but who may live around people who do, and in that point of view I almost want a warning, so these persons know they have to be careful when they read.
So at the end I did mention it in my summary, but in a way I find pretty ridiculous even if it may do the job?