How to bring up sex with a defensive woman???

First time poster, long time reader. Sorry such a long post but I need someone to talk to about this. Any time I try to bring up the subject of sex or my feelings about sex with my wife, she gets very defensive and angry. Then her first reaction is to tell me I don't respect her feelings and then I get the silent treatment and all affection stops. This usually lasts until I apologize. I try not to let things bother me and keep my mouth shut. As long as she thinks I am fine with our sex life then everything is calm and normal. At least normal as she sees it. My issues are basically simple. At least I think so. See, I feel depressed because of a lack of affection and I feel like she does not desire me at all. We do have sex. About 2 times in about every 3 weeks. This is okay, but when we do she mostly lies back and lets me do all the touching I want but barely touches back until she gets closer to her release. then it seems rushed to get done and that's that. Its not really the quantity of sex we have that is an issue for me but more so the fact that I don't feel desired or wanted sexually. I know she loves me but I feel its more like really close friends than lovers. We have been married for over 20 years and have 3 kids. they are 20, 18 and 11. Our sex life has been like this at least 15 years if not longer. Every time I try to spice things up or try to ask for more from her she gets mad and tells me that I know how she is and I need to respect her feelings more. I have been having performance issues for the last couple of years now and she says its because of the amount of coffee I drink and the fact that we both smoke. I however, feel it is caused by the fact that I don't feel wanted. but I reason with myself to leave it be because some sex is better than no sex.

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