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.
Feelings aren't instinct. (Feeling disgusted or sexually aroused or aggressive isn't an instinct.)
Instincts, by definition, are complex behavior. (Think birds building nests; nobody teaches them to do that, they are born with that knowledge.)