Pending any evidence to the contrary, this is most likely how the minister discovered the problem in the first place.
It probably was how the minister found out. The prospective member stated they were homosexual. Now, where the conversation went from there is something we are all speculating.
If the person confessed homosexuality as a sin in this disclosure, then I believe it was wrong to deny membership.
If they informed the minister as an "in your face, you must accept me as a homosexual because I see nothing wrong with it" then I support denial of membership. No different than someone wanting to join the church, but insisting they were going to home to worship Satan after service Sunday and found nothing wrong with it.