I’d say at face value, they’re not lustful. Lust is a desire to use the other person for our own sexual gain or pleasure, and butterflies are simply feelings we get when we encounter someone to whom we’re attracted. I think that if you, as you say, turn the experience of those feelings into an “unhealthy infatuation,” then it could turn into lust. If you do encounter someone who’s really attractive, thank God for their attractiveness, tell Him He did a good job, and move on. As a guy, I can tell you this is really important and effective in making sure we respect and love women rather than using and lusting after them. Hope this helps.