Is sexual arousal by some people's feet (only some) a mental illness? if so,that would mean psychotherapy could make those thoughts go away, right?

Feet: It is possible that sexual arousal by feet is a paraphilia as defined in the Diagnostic manual the DSM5. BUT it might not be because it is a complex diagnosis therefore you must see a psychologist or psychiatrist for an evaluation and diagnosis. Be well, Dr.G

Only if distressful: The wide range of situations, things, body parts, thoughts that people find sexually arousing is well-known and part of human nature. The issue of whether or not it requires treatment depends on whether or not it is distressing to you and/or others around you.If so, a psychotherapist may help you to address ways to change behavior or thoughts to reduce or eliminate such distress.