Richard Page was suspended as an NHS Trust director after he claimed it was better for a child to be brought up by both a man and a woman.
Kent and Medway NHS and Social Care Partnership Trust (KMPT) said that his position on the matter "undermined" the confidence of the non-heterosexual staff.
Also, in 2014, while working as a magistrate on a family panel he opposed a same-sex couple's application to adopt a child. He told the hearing that it was "generally in the best interests" for a child to have a mother and father. The court clerk and two other magistrates then lodged official complaints.
Mr Page, 71, has come under heavy public criticism.
The Lord Chancellor at the time, Mr Michael Gove, condemned him for being influenced by his religious beliefs rather than evidence. He was also involved in a public spat with TV presenter, Piers Morgan, who accused him of being a homophobe, a claim Mr Page refuted.
Andrea Williams from Christian Concern, which is supporting Mr Page's legal action, said: "This case is another in a long line of cases that demonstrates the intolerance of our illiberal elites. Far from promoting diversity they punish people like Richard who serves his community so well."
Laws allowing gay couples to adopt were introduced in 2002.
The tribunal hearing will be held at the Croydon Employment Tribunal and is expected to last four days.