Transcript for 'Sister Wives' Daughter on Coming Out as Gay to Her Family
When you have 18 children and four different wives, it's likely one of them will do something to shake the foundations of your belief system. What was that it stunned this polygamist? Here is ABC's Abbie Boudreau. Reporter: Life for polygamist Cody brown, his four wives, and their 18 kids, all stars of tlc's hit show "Sister wives" was already full of drama when oldest daughter Mariah recently dropped this bombshell. Whew. Okay. Um -- I'm gay. Oh, honey, I thought you were going to tell me you had cancer. Are you serious? It's one of those things I had to say it where I wasn't going to. I was just like, okay! Reporter: A shocking and unexpected revolution for the high-profile "Sister wives" family who believe being gay is against their religion. I had no idea Mariah was gay, no idea at all. It was Mariah's biological mother, Mary, who seemed the most stunned of all four sister wives. You actually teared up. I think it was just such a shock. And completely different than what I had ever expected for her life. I realize now that it's okay for me to mourn what I thought was going to be her life. Reporter: Also shocked, the outspoken patriarch of the family, Cody. Mariah never dated boys. A fundamentalist Mormon culture, the polygamists, they don't date for fun, they date for courtship, they date with intention to get married. So we just thought maybe she was just waiting for that person that she might have met. Reporter: The 21-year-old says she was fighting the truth about her sexuality for years. It wasn't necessarily me keeping a secret, it was something I couldn't accept in myself. Reporter: Mariah says her parents preached acceptance, both at home and in public, but growing up in the church, she feared the repercussions of coming out as a lesbian. The people I was around and the church I went to, it was very -- bad. Like -- gay people are bad. It was just like, I can't be perfect in the eyes of god unless I'm not gay. Reporter: Cody is quick to protect his daughter, even in the eyes of his church. This is my church and my faith and I have chosen it. Okay? She doesn't have to choose it. Even though you're accepting of it, do you still feel like being gay is a sin? I went to god myself and asked him what to do about it. He said, you love, I'll judge, and guess what, that message to me was, it is off my shoulders, I do not have to worry about it, my job is to love. Reporter: Suppressing her feelings for so long, Mariah says she became homophobic. I think I didn't want to be around it because -- I guess I wanted it, you know? So when I did start being around people who are gay, it was like, you know what? They're actually fine. It's okay. Reporter: Citing opposing beliefs, Mariah left the church, but it wasn't until college that the psychology major finally allowed herself to stop living a lie. Was there a certain moment? I had been talking to one of my friends. She told me she had a girlfriend. And I was like, oh. She's gay. And I immediately after that was like, oh that means I could date her. Then I was like, wait, what? I don't want to date girls, that's weird, I like boys. Reporter: Mariah's announcement came in the midst of a rocky time in her parents' marriage. Is that what you're waiting for me to do is come crawling back on my knees? I've never said that. I'm asking. Reporter: Meri was the first to wed Cody 27 years ago and says she was supportive when he then married Janelle, Christine, and robin, and had 17 more children. But it is the bond with his very first wife that eventually became the most strained. I just don't know that you and I are even on the same page. Are you still in love with Cody? We don't have the romantic love that we did at the very beginning. We don't. But I definitely love him and I have a lot of hope for our relationship and our future. How would you answer that question? You know, on a practice tonic level. I mean, we're just not in a loving relationship. Reporter: Despite being surrounded by her large family, meri says she felt lonely and vulnerable when she found companionship online. It did feel good to have somebody that I was, you know, talking to. And that seemed interested in my life. Do you feel like you were having an emotional affair? No, I don't. No. Do you feel like she was? I don't now, but at the time I thought she was having a full-blown affair. Were you angry with her? I was worried. We were worried and angry and sad. Confused. Yeah, confused, a little bit depressed. She was so checked out from us. Reporter: Yet meri was checked into her online friendship. They says things took a dark turn. That's when all the threats were happening. You know? Stay away from your family, you don't want to be with them because they're bad to you. Reporter: Says it was months into the relationship before she realized she'd been catfished. By a woman posing as a man. If you and I had been taking better care of our relationship, then I wouldn't have been in such a vulnerable place that I would have been open to other friendships that would lead to this kind of deception and evil. What do you think the end game for the catfisher was? Attention. That's it. Attention. Money. I don't know. Reporter: Devastated, her family rallied around her. But her relationship with Kody still suffers. Do you trust her? You know, I trust her on a pragmatic level. I don't trust her emotionally. Is divorce on the table? Not for me. Um -- no, then. It's not on the table. Because I don't leave. I'm with these four wives, and I don't leave. Reporter: They say after years avoiding the truth, they will no longer lie to themselves. Mariah too is living her life in the open, even though she isn't dating yet. I can't believe I'm about to ask you this. Do you have a girlfriend? No. Reporter: She's thankful for all over parents' unconditional support. I'm really proud of Mariah. We love her. It's not something that changes anything the way I feel about her. Reporter: And her siblings as well. When Mariah told me she goes, I wonder who would be -- who of us would be gay. Do you feel like you're happy? Yes, I'm happier than I've ever been in my life. A weight off the shoulders. Yeah, it really is, seriously. Reporter: For "Nightline," I'm Abbie Boudreau in Las Vegas. "Sister wives" airs Sunday night on tlc.
This transcript has been automatically generated and may not be 100% accurate.