The Baha'i law stating that marriage is only allowed between a man and a woman does not make the Baha'i Faith homophobic.
Only people can be homophobic.
The reason for that law is because Baha'u'llah stated that the primary purpose of marriage is to have children, and last I checked it takes a man and a woman to produce children.
homophobic: having or showing a dislike of or prejudice against gay people.
homophobic means - Google Search
Baha'is do not have a dislike of or prejudice against gay people.
A Baha'i law is not homophobia, except in the distorted minds of people who want to denigrate the Baha'i Faith.