found this, although I don't put too much stock in it
http://churchmilitant.tumblr.com/sexquestions
4 - Are both spouses guilty of a mortal sin if one chooses to use a contraceptive against the others will?
No. If a man decides to have a vasectomy against his wife’s will, she may still have sex with him and not be guilty. Likewise, if a woman is taking birth control pills against a husband’s wished, he may still have sex with her and remain guilt free.
Pius XI teaches the same thing in
Casti Connubii:
"59. Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order. In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin."
http://www.vatican.va/holy_father/pius_xi/encyclicals/docuмents/hf_p-xi_enc_31121930_casti-connubii_en.html
Notice an effort to correct the spouse must be made, even if it doesn't succeed. If there is no practical change, keep praying that she has a change of heart. There's nothing more you can do. But there's no obligation to abstain.
If your conscience still troubles you, talk to a priest for reassurance (don't go near a Novus Ordo priest, and find an absolutely traditional one who teaches that NFP can only be used for a grave reason). He will tell you the same as Pius XI, but it often helps to have a priest's personal assurance that you are doing nothing wrong, and he may pray for your wife as well. Have a Mass said for your intention (that your wife stops using this method).