Perhaps, the couple seen in the trailer who you play one half of are NOT actually married. They could infact be in-law siblings who became close friends, having met through their married siblings. As for the baby, s/he is the child of that couple and not the PC couple, but has some resemblence to the PC couple through their brother and sister. As for the baby's real parents, they caught New Plague and had to be put under quarantine. However the baby was tested and found negative for New Plague and so was put in the care of their next of kin, who were also chosen because of their proximity to a Vault. Here's a family tree to help.
Fem!PC - Baby mummy = Baby daddy - Guy!PC
_____________________\ ____ /
______________________Baby
And then Fem!PC and Guy!PC adopt Baby after his/her parents catch New Plague.
Now I'm not suggesting this is how it will be written, I'm just suggesting a bit of creativity for the benefit of us gamers who want to play this game badly but as a homosixual PC. Sound good to anyone else?