Dito what everyone has said here. I would first tell your wife what you just told us...that you love her very much but that you have a desire to improve your sex lives and intimacy. Then go see a counselor with or without her. Stress to her the importance that she go with you. Don't accuse, blame or give ultimatums that will shut her down. Let the counselor find out what and where the problem is.
Also, as The Fuse mentioned it could be medical. Encourage her to see her OBGYN. That may have to come after counseling since she may not think anything is really wrong and be hesitant to ask her Gyno about "sex drive"
DON'T get into the lifestyle alone though...that is cheating and you sound like you love your wife. Work on the marriage. If you hang around here and read you'll find the majority of the couples had a great marriage and sex life before the lifestyle and this is just an added recreation they enjoy together.
Good luck
Mrs Spoomonkey