Wow, well, is she particularly stressed? If so, you could take some of the burden off her by offering to say.. help around the house or something. She may be someone who is not comfortable with sex in general, maybe she was brought up thinking it's 'bad' or 'dirty'. If that's the case, she may require some counseling or something. Maybe you could get her a subscription to Cosmo, she could read about all the 'fun' she's missing out on by having only 'vanilla' sex. And lastly, does she have any medical problems? Some medications, including for instance anti-depressants, can lower libido. If she's on a med that can have this effect, she may want to change her RX. If it's not a med, she could speak to either her family doctor or gynecologist to see if they have any ideas if anything medical is wrong. Good luck!
Edit: Oh, and I agree with the other people who said to wait before trying for a baby. You definitely don't want to bring a baby into this until you know your relationship is more stable.