Easy Answer, Puck

It is because they don't have any gay children that they are so pro-gay.

Okay, so that isn't true. However, they feel that they can win more "points" by pointing out the GOP's supposed hypocrisy ("They have gay kids that they claim to love but won't push the Gay Agenda (TM)"). The upshot of this, prepare to be stunned, is that they don't care about homosexuals or their problems per se but merely see them as a way of pushing their agenda of the week.

Because the Dems like to think in "groups" (all x are alike and have the same goals) they put people into their groups and treat all members of that group not as individuals but as part of that group (follow me?). Therefore, all gays want the same thing (gay marriage in this case) and since Mary Cheney is gay she wants that and if her father loves her he'd want that too.

Or was your question rhetorical?