By Dr. Jeff Nichol/For the Journal   Q: What do you do about a cat acting out sexually? We've had Sebastian "Basti," 20 months old, for a year. He was neutered at eight months. He's very sweet, but recently has started humping my bedding, my clothes and he attempts to hump me. He bites the object on its surrogate neck and does pelvic thrusts and butt flutters. Frankly, it's off-putting. Basti gets along with our other two cats. They climb trees and play in our large yard. Dr. Nichol: Butt flutters, eh? Ahem. A delicate subject indeed, although not for...