Utah laws Title 34A (Utah Labor Code), Chapter 5 (Utah Antidiscrimination Act)
Section 106 (Discriminatory or prohibited employment practices) provides that:
(1) It is a discriminatory or prohibited employment practice to take an action described in Subsections (1)(a) through (g).
(i) An employer may not refuse to hire, promote, discharge, demote, or terminate a person, or to retaliate against, harass, or discriminate in matters of compensation or in terms, privileges, and conditions of employment against a person otherwise qualified, because of:
It does not say "the employee's religion".
However, under subsection (3)(a) of section 106, if religion (or any of the other categories protected under subsection (1)(a) is a bona fide occupational qualification, it may be the basis of a decision to hire, fire, or other employment actions.
Section 106 does not apply to religious organizations, in any of various organizational formats, according to section 102. An employer is defined in section 102(1)(I)(i) as:
a person employing 15 or more employees within the state for each working day in each of 20 calendar weeks or more in the current or preceding calendar year.
(In addition to various governmental organizations)
Nothing in section 106 specifically mentions discrimination because of the religion of a spouse or dating partner. But it does say "because of religion" without saying whose.
This Utah Government site has addiitonal information.
Note also that even where it is found that an employer did not in fact unlawfully discriminate, if an employee complains about what the employee believes in good faith to be unlawful employment practices, and the employer takes adverse actions because of such complaints, that may still be unlawful retaliation. The case of Viktron v. Labor Comm. Case No. 20000386-CA (2001 UT App 394) deals with such a case. Other sources have suggested that retaliation is sometimes easier to prove than the initial act of discrimination is.
Most large multi-national companies have internal anti-discrimination policies, often ones stricter than any law requires. The one i work for makes it very clear that any such action should be reported to HR or to a designated high-level office, and promises that serious action will be taken against managers enraging in such discrimination. It has even distributed videos in which various employees describe how they reported such improper practices and how the company responded.
Of course, I don't know how your former employer's policies on such things read, or how they are enforced in practice. But it might be that going over the head of the supervisor would resolve this without legal action. In some cases, publicity can induce a company to "do the right thing" when it might not be legally compelled to.
This story in the Salt Lake Tribune Discusses the UALD and the state of enforcement of the law.