This would be fairly recent - within the last 20 years or so.
A woman researcher suspects that there is an alien species living among us. They are beautiful, unattached young men, not hostile or violent. They stay awhile in one place, work temporary jobs, and sooner or later move on and disappear. Their girlfriends, of course, get pregnant.
There is no guarantee that this is any sort of conspiracy on the part of the "boys", although the scientist believes this is the case, and they may even be a human subspecies which has evolved to take advantage of a social/ecological niche. They only make up about 2% of hunky young men. Since they are biologically identical with humans, identification is based entirely on sexual attractiveness - which has certain obvious flaws.
The researcher eventually falls for one of her subjects. Who disappears after she becomes pregnant.