So, you live in a Mormon family, not just a Mormon family, but a Mormon family that practices polygamy (I understand that the Mormon Church doesn't approve of the practice, I'm not suggesting they do). You're the daughter of the first wife who was the only wife for quite some time. You're uncomfortable with the very notion of polygamy and more than a little perturbed at your mom for allowing your dad multiple wives.
Polygamy aside, your parents are quite religious. Sex before marriage is a big no-no, the very idea that you might be on the pill has practically caused your mom heart palpitations. Man, if only she knew that your weren't on the pill (she didn't believe you when you suggested it), that you were instead knocked up. Yeah, that wouldn't go over very well, in fact, it might get you booted from your family.
The way I see it, that might be why you allowed yourself to get impregnated – the notion that your ties with your parents could be severed by them and not by you was, subconsciously, an attractive idea. Obviously you would still be somewhat culpable for the ending of the relationship, but it would still be your parents that initiated that final confrontation.
Such has been the lot of Sarah Henrickson on Big Love this season, and it's been the most compelling storyline the family itself has had to deal with outside of the Juniper Creek hard-line Mormon compound in a long time. Of course, the biggest problem for the show was how to have the storyline progress. Sarah could always opt to tell her parents, get booted from the family, and the show could then have her go out on her own for a while. Doable certainly, but that would only allow for a moment of conflict. Having the baby in the bosom of the family doesn't seem like it would be true to the characters. What then?