Sexual arousal is part of the procreation mechanism, and thus I (not a psychologist) will classify it as instinct. I can think of many different feelings as instinct. When I see my gf crying, I don't 'think' to hug her, I instinctively hug her. When your friend walks two steps ahead of you and falls down, you don't stop to think and feel, you instinctively grab them.

An instinct is complex, unlearned behavior.

Sexual arousal is neither complex nor behavior. It is a simple hormonal state.

You're using a colloqual meaning of "instinctive" that is unscientific and wrong.

Encyclopedia Britannica [1] also doesn't agree with you. > ...it is taken to include the possibility that humans too can be governed by instinct

But that's far from being the only source that defines 'instinct' in a way that doesn't align with what you're proposing.

Humans do have instincts.

[1] https://www.britannica.com/topic/instinct/Instinct-as-behavi...

Yes, many things can be "taken to include the possibility".

Including the possibility that Gandalf and Harry Potter fly in on a Raelian flying saucer and solve climate change for us.

In reality, however, no instinctual behavior has yet been found in humans.