It's normal. I just replace someone I care about in their spot. But when it comes to weight I imagine someone I actually have a crush on feeling me with those pounds.
Finding others attractive is perfectly normal. My wife and I have been together over 22 years. In that time we've never cheated or strayed. Nor have we even wanted to. But, seeing someone else as attractive or sexy is fine. Its about actions. If you don't pursue any of those feelings I don't see how anyone can fault you for having them. I'm sure its fine.