Wow, that, for me, is a big red flag. If she's essentially stating the only thing that's going to get her back in bed is rape, her mind is really afield. No one in a marriage, who really loves the other person, should be so against compromise that they feel the only way they will do something is essentially forced by violence. Marriage is about intimacy, and even if she's not super excited about sex itself, she SHOULD be excited about making you happy. And that's the big thing; if she can no longer engage in something, even if its just for her partners happiness, because her partners happiness means so little to her? That is the issue.
I've had sex with my wife when I wasn't fully in the mood (It was enjoyable, I was just not in the right state of mind--but she wanted to, and it made me happy to make her happy.)...And my wife has also done that for me. Relationships are built off these things, it's not all about what YOU get out of the act--if you love your partner, part of it must be about what THEY get out of the act. If she can no longer find it in her to derive happiness from helping you? Then something is wrong.
I would ask her to talk to a doctor honestly...if you had a healthy sex life before, and things went off a cliff, it might be medical. So, rule that out before you push her on the emotional aspect of it.