Lust is always sinful, whether the object of it is alive or dead. But "lust" is not just "sexual thoughts". I don't know your particular denomination, but the Catechism of the Catholic Church - which I hold to be the true Church insofar as doctrines differ between denominations - teaches the following: >2351 Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes. I would say fondly remembering the enjoyment one shared with one's wife before her passing would not be sinful, but any *prolonged* dwelling would probably begin to constitute something "inordinate" or "disordered". To think on the pleasure only, and not its end, would also probably constitute lust. If one is getting aroused thinking about it, it is also probably disordered - since the unobstructed act itself, with her, is no longer a possibility. If you aren't just speaking hypothetically, then I'm sorry for your loss - may your wife receive her eternal reward. I will pray tonight for her soul, and for you in your mourning.