Does the concept instinct apply to intense biological motivations . . . ?
Yes, within the proper context. Human beings automatically know that pain should be avoided and pleasure should be pursued, with the exception of those who have deliberately trained themselves to feel otherwise.
So for example, when an infant’s hunger prompts it to cry, the emotional response itself could be considered instinctual (which is precisely why I consider certain values axiomatic). The act of crying itself would not be.
for example, can one say that the intense biological attraction towards a potential mate/companion during adolescence (lots of hormones) is “instictual”?
No. Sexual attraction may seem axiomatic but it is not.
Think of the reflex to kick, when your kneecap gets tapped. That’s an instinct. So is your attitude towards hunger and thirst. Whatever is instinctive is on the sensory level of awareness.
Now notice that sexual attraction depends on the perceptual identification of someone (as opposed to a tree or a table, etc.), and that not just any human being will suffice. Observe that the traits which any given person finds desirable are different from what any other person may value, and compare such differences to the moral standards behind each one. . .
Sexual preferences are absolutely meant to determine the ‘fitness’ of one’s mates, in the fullest sense of the term.
Is there a better way to characterize it?
Yes; an accumulative choice. Any person’s sexuality is dictated by countless, implicit judgments and evaluations, concerning what people should be like (look like, think like, etc.); you choose your own desires over many years’ worth of thoughts and feelings and your internal reactions to them.
I honestly do not know what role hormones play in this process, but I do know that hormones do not dictate our emotions; they are our emotions (in the same way that synapses are our thoughts).
And I also know that the development of the sexual capacity, for most people, coincides with the burning need to form some coherent view of life.