The Christa being pregnant thing is an interesting theory, but I'm not sure if it sets well with me. Number one, Omid is the man for doing the horizontal monster mash with her during the zombie apocalypse, but since she's skinny as a twig (No signs of pregnancy) this had to have happened fairly recently (Since so much time has passed from the beginning of the ZA). That just seems like a risk most people wouldn't take. If this happened before hand, she'd be showing already. Just my thoughts.
If we had no place to stay, were on the move, and so forth, "Hey honey, let's screw." would be the last thing on my mind. Unless, of course, she was a rape victim, or something, but I think that would play out differently. I just think they have an ulterior motive with the group, for the moment.