You mean why they wouldn't just ask women if they've faced domestic violence, and if so by whom? I see what you mean, but this gives us more information, particularly about same-sex attracted people. Living situations are more straight-forward than sexuality because people's acceptance and awareness of their own sexuality can change, e.g. I know lesbians who thought they were straight or bi, then realized they were lesbians. So this accounts for same-sex attracted people in an objective way (you either live(d) with your partner(s) or not), rather than relying on self-reported labels that are subject to change depending on awareness/acceptance.