Our society heaps so much guilt and shame onto sex and sexuality, but a writer who breaks past that to celebrate and empower their own sexuality can empower their readers to do the same.
This excessive concern over a minor matter sounds like the product of an anxious aversion to declaring the book finished.
The question is how to go about writing characters who aren't like you, not whether you are allowed to.
It's challenging to have the desire to create works that cause minimal harm, paired with the certain knowledge that our writing incorporates our ignorance and erroneous beliefs.
Romantic tension is one of those things that's often better managed during revisions, because it's all about pacing.
Craft your HEA to fit your characters. Let them surprise you with the innovative solution they all come up with.
Characters take an approach of "How can we make this work for all of us?" rather than "It's too bad I have to choose one of you."
You are totally permitted to just do the fun parts of this and skip the parts you dislike. Hobbies are for enjoying.
Don't try to tell an "authentic" story. Instead, tell an "inauthentic" truth.
Let somebody else write only allistic characters. You can write a character who is probably, or maybe, on the spectrum, but this does not need a huge reveal or internal struggle—it can just be a thing you do, among other things you do, as an author.