I am and always have been in favor of "something for everyone," though whether or not that philosophy should apply to any given game is a matter for the developer to decide. I don't think that every game needs to cater to everyone. But I have no problem with Femdom, or any other fetish which I don't personally enjoy, being included in this or any other game, so long as I don't have to play through it.
That being said, I'm also someone who usually tries for 100% game completion. And as for Diane being avoidable if you don't read the emails, reading the emails is part of advancing the Angel storyline, unless I've misunderstood. So, wanting to advance that story, I have the MC read the emails. If Femdom is going to necessarily follow from that, I may have to rethink pursuing the Angel storyline.
I realize that it would be a lot of extra work for Faerin, but my preferred solution would be to have diverging paths for Diane, one submissive and one dominant. Heck, if catering to people who like Femdom is the consideration here, the same could even be applied to Veronica. But if the only path for a given character, like Diane, is Femdom, then give me an "abort" switch once I've encountered her, so that I can skip that content. Either that, or give me the chance to flip the script on her fairly quickly and take a dominant path with her.
I'll wait to see in which direction Faerin decides to take it.