"For Catholic educators to suggest that Church teaching is in error is misguided and offensive," the alumnae wrote.
"Sexual union in marriage is only one among many possible paths to a life full of love; human dignity does not depend on sexual expression, and it is perplexing to hear a professed religious sister insinuate otherwise."
The open letter accused Berchmans of "a heartbreaking betrayal" of the Salesian order's founder by using quotes from St. Francis de Sales in an argument which seemed to pit the Gospel imperative of love against the Church's teaching on same-sex unions.
The alumnae said Berchmans implied that those who affirm Catholic teaching "act out of hate."
That implication, they said, would be an indictment of Pope Francis, St. Francis de Sales, and millions of faithful Catholics around the world.
The letter's four signatories said they had been overwhelmed with private messages of support from other recent Visitation graduates who shared their concerns but believed they would be "rejected and condemned" if they came forward publicly.
Before the open letter was published, CNA spoke to several parents of current students who voiced similar concerns. Those parents said they are concerned that the school's decision to publish same-sex union announcements is part of a growing pro-LGBT agenda within a small section of the school community.
One father explained to CNA that he believes the new policy for the alumni magazine would serve as an example for other Catholic schools to break with Church teaching.
"This isn't just being watched by the immediate community. I've spoken to parents from other schools concerned this could be the start of a national trend [by Catholic schools] away from the Church and towards a progressive agenda," he said.
"This isn't what we want for our daughters, we make sacrifices as a family to get them to a school where the faith will be taught and nurtured, not undercut by the administration."
One mother told CNA that the alumni magazine decision had crystalized growing concerns among the wider school community.
(Story continues below)
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"This isn't about one or two or ten families taking issue with something in a newsletter," she said.
"A lot of families - a lot of us - have been concerned for a while now about a real move away from a truly Catholic identity to something just 'in the Catholic tradition,' and that's not what we signed up for."
Several parents told CNA that they have been in touch with the Archdiocese of Washington asking for newly-installed Archbishop Wilton Gregory to review the situation. While the school is under the direct oversight of the Salesian Sisters and not the archdiocese, the local bishop has a general responsibility for ensuring that all Catholic schools are faithful to Church teaching.
On May 15, a spokesperson for Visitation told CNA that "I can't speak for the archdiocese, but I can say we have been in touch with them and our goal is to work with them as we move forward and remain committed to our Catholic identity."
An official spokesman for the archdiocese declined to comment on the situation.
CNA also confirmed that Bishop Michael Burbidge of Arlington, Va., had written to the school's administrators expressing the concerns brought to him by Visitation parents living in his diocese.