Washington law provides a specialized definition of "animal" only for this specific law, necessary since humans are animals, and the goal was to limit the scope of the no-sex prohibition. "Human being" is not defined in Washington law, in fact it is the undefined basis of the term "natural person". At the federal level,
the words “person”, “human being”, “child”, and “individual”, shall
include every infant member of the species homo sapiens who is born
alive at any stage of development.
which is to say that the federal government solves the problem by reference to a species classification. Washington state does not do this.
Whether or not you and Deanna Troi can legally consummate your relation in Washington is a somewhat open question, insofar as you could do it, the county prosecutor being a xenophobe could decide to make an example of you so you get prosecuted and maybe convicted. You nevertheless have an option (if you get a decent lawyer), which is to argue that she is a human being and that the jury is to be so instructed. This will end up at the Supreme Court (at least Washington Supreme Court), in the unlikely case that Washington SC does not decide in your favor, you take it to SCOTUS and hope for the best.
If the final judgment is that an alien is "not a human being and not homo sapiens", you will know that your act was illegal, and furthermore that Deanna Troi does not enjoy the rights of (other) humans. Supreme courts are constantly discovering subtleties about the legal meanings of words, and since this is a question that bears on a fundamental constitutionally-protected right plus it involves a suspect classification, the interpretation of the law will not be made according to the loose principle "the government can do whatever it wants". Instead, the law will be subjected to strict scrutiny ("Suspect classifications include race, national origin, religion, and alienage"). This makes it vastly harder to apply this law to sapient volitional species.