I'd agree that the obsession with bloodlines may be mostly a thing among the upper class. For the lower classes, the main purpose of marriage and procreation is probably to ensure you have someone who will take care of you and your lands and/or business when you age or become sick or injured, instead of having to hope for charity from non-immediate family, neighbours, or the Chantry (or other applicable belief system). An orphan will do just fine there, and adopting it is a service to the community too, so I'd be surprised if at least some societies and traditions didn't look favorably at it. Same for taking in a kid from a family with too many mouths to feed. And of course, one or both women in a same-sex couple might try to get pregnant themselves if they can stomach the notion of "doing it" with a suitable man. A male couple would have a harder time ... I don't think many women would be willing to risk their lives incubating and birthing a kid they don't want to keep.
It's also possible that a younger sibling may stick around to help with a larger family farm/business anyway, helping provide for the heir's children and being taken care of by them later in return. In that case, they could hook up with a partner of any gender and still be productive members of the family without the expectation of having to breed. They might even be discouraged from doing so to avoid the "too many mouths" problem that could ruin the family or break it apart.
Basically, I'd expect a diverse setting to have varied views of this sort of thing. Tevinter isn't Ferelden, Orlais isn't the Avvar, Rivain isn't Orzammar, and even Dalish clan A isn't Dalish clan B -- if all these cultures differ in other ways, there is room for some differences in their views of partnership and procreation, too.