Most books traditionally are multigenre, actually. But one genre dominates. So there may be a sci-fi romance, but either the sci-fi or the romance dominates. Why? Because they have to be able to shelve the book. If it’s so much of a mashup that no one genre stands out, trad pub won’t buy the book.
If you self pub, it’s irrelevant. You won’t be on store shelves, and you can “shelve” it multiple places on Amazon.
As far as comps go, you’re not looking for something exactly like your book. You’re looking for books with a similar premise or similar themes or similar tone. You’re looking for authors who write similar books, whose readers would like yours too.
I agree with @Tirellan – if you can’t identify comps, you’re likely not reading widely enough in your genre(s).