The child of a politician is generally off limits in political discussion, unless they themselves get involved in the debate. Mary Cheney was involved directly in her father's campaign, serving as the director of vice presidential operations for the Bush-Cheney 2004 Presidential re-election campaign. Her direct involvement in electing a man whose politics are DIRECTLY OPPOSED to her lifestyle makes her fair game in political discussion. She IS a political figure.
Discussing how her father feels about her pregnancy, when her father's big supporters claim that homosexual couples should not have children, is absolutely acceptable.
She made herself part of the debate. Hopefully, by the time her child is old enough to understand what's happening, the whole issue will have blown over and been resolved. But at the moment, her life is fair game. She involved herself in politics, as part of a party that HATES her and everything she stands for. As long as she supports Republicans who think she's morally deficient for simply existing, her life is part of the debate.
If she has a problem with that, maybe she should've thought twice before working to elect people who think she's a second-class citizen.