Definitely a valid concern that guys don’t have to worry about. When I was interviewing for my first job after leaving the Navy, I was pregnant but not yet showing. I know they’d have never hired me if they’d known. Lucky for them, I NEEDED the job and literally worked up until the day before my daughter was born, and I was back in the office in 3 weeks because I didn’t have any sick time left. So being female, millennial or not, has additional issues in the workplace.
That aside, I don’t see a problem with finding out what an employee sees in his or her future. An employee who likes his job, does it well, and really isn’t interested in moving up a corporate ladder can still be a valuable employee. Unless you have a boss like I had once. When I told him I wasn’t interested in following a management track, he proceeded to tell me why I was wrong. I was so glad when they had a reorganization and I was no longer working for him.