Frogskin,
I know how you feel. My wife was not into it, despite my constantly raising the issue during sex. She just wasn't into sex with someone, it turned out that she needed an emotional and intellectual connection with a guy. That happened and the sexual relationship blossomed. Now she fucks him every few weeks (he lives pretty far) and she routinely flirts with other men. Her sexuality has emerged into a full blown cuckoldress. Try finding someone who will romance her and not just look for sex and you will get your wish. Beware, the emotional connection can be strong and your marriage must be strong enough to sustain a it.