On the one hand, if I keep them under one name, then that's just one mailing list, one website and one Facebook fan page etc. But I don't think people interested in romance erotica, would be all that happy with M/m or something similar.
Actually, a sizable percentage of M/M erotica writers and readers are women. Who I would assume also consume romance erotica. But I understand that's only being used as an example.
If you wanted to write both erotica and non-erotica, that would be a separate animal all together. But this is just within erotica. You certainly
can go the separate names route. Others have. But I think that you'd do yourself a disservice by creating a pen name for every different varied work. Once fans find out you write good, they are going to want to read what you write. It's your writing style, it's the quality of your writing, it's your characters, it's just that you produce good stuff. That's why someone would come back to You - an expectation of quality. So by you using a separate name, those fans won't be able to find You - you're starting all over again at 0.
Yes, not every fan is going to like every kink or orientation or subgenre of erotica you write. But neither is every fan going to like every Story you write, either. There are authors I follow that sometimes write stories or books with subjects, genres, etc that don't interest me. I just skip that content and wait for the next thing that I like.
The concern is that someone is going to assume that your name = stories about x. They automatically read what you write because it's you, and since they don't like x, they'll get upset. Well, yeah, that can happen. The best way to reduce the chances of that is being up front about the contents of a story. Put tags on the book that is clearly M/M erotica. Say in the blurb, "This is M/M erotica." Then the only people who get upset are those that don't pay any attention to anything. And if you're worried about upsetting readers/pushing people away, that is an inevitable thing. You can't please everyone all the time.
Not only is it more stuff you have to juggle, but you're also considering radio silence. Let's say you have 3 separate names, one for M/M, one for normal romance, and one for BDSM. If you write a story for each one, then people following you for say, M/M are going to be waiting 2/3rds times as long just to hear back from you, to hear what you're upto, to see that you're active and putting out new content. Even if you're not putting out content that the M/M crowd is interested in, they will see you're not dead.