Homosexuality is a chosen behavior that is contrary to the fundamental unchanging truths that has been ordained by God in the Bible, recognized by our nations founders, and shared by the majority of Texans.
In that sentence, the verb form has (rather than have) necessarily refers to a singular subject. The only word that qualifies is not the plural “truths” but the singular “homosexuality.” Compounded by the unnecessary confusion over a relative clause (there are two phrases beginning with that, each referring to different things), the sentence ends up alleging that being gay is “ordained by God” and “shared by the majority of Texans.”
As a famous Texan once said, “Oops.”
Also, “nations founders” is missing an apostrophe in the first word; it’s a possessive, not a plural.