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It may be due to the fact that she was abused by her step father and it didn't come out it until she found someone that truly loved her, itcould be the fact that before she associated sex with something bad and maybe her ex wasn't the nicest person to her and didn't treat her the way you do with such respect and so loving. She doesn't have anything to prove with you and feels a sense of security her step father and ex couldn't give her and she doesn't feel like she has to justify herself through sex she may also have just started feeling the effects of the sexual abuse inflicted on her by her step father and she may need to seek some type of counseling for that abuse. She needs to work out her issues as far as the abuse because I think this is stemming from her abuse and has nothing to do with you and your sense of security you give her makes her feel like she has nothing to prove to you and that she doesn't have to have sex with you as much as she did with her ex or maybe that sense of security brings back the memories of abuse because she was supposed to be able to feel secure in her own household and her step father took that away from and she is associating that with your relationship.
Your only choice may be to get her counseling where she can work through what happened to her and not blame herself and to blame the sick person that took advantage of her as a child, the counselor can also help her to forgive what happened to her and feel like she deserves happiness in her life. She may be blaming herself for what happened but what she needs to understand is that she was a child and he was the adult and should not have taken advantage of her in that way. She has kept this secret throughout her life and kept it bottled up inside of her because she was ashamed of what he did but she isn't the one that should be ashamed her step father should be. Did she prosecute him when this happened? If she didn't tell and kept it to herself or if nothing happened to him this may make her feel like she will never be secure and that the security you give her is false security but you won't know the truth length of her torment until she gets outside help. Just try to be supportive and be there for her she will come around with help.
There needs to be a compromise between the two of you and she may have way more issues stemming from all of her relationships than she wants to admit this is why I said she may need more counseling there is a reason that you feel like you are married to all three of those men because all three of those men hurt her in a way that she still has not been able to get over. The step father made her feel dirty, the first husband broke down her self esteem, and the third husband (Who is not gay) made her feel inadequate and now you are taking the brunt of that hurt. If you start to affirm her she will only feel even more inadequate then she had before and she will wonder why you have changed on her also and she will think it she that there is something wrong with her.
Your wife really needs counseling of those relationships she has had with men have been negative and now she seems to feel like she isn't worth anything and that is why she isn't giving you the intimacy you are desire because she doesn't feel good about herself no matter how many times you tell her she is beautiful and sexy or looks good she will always find things wrong, she has to learn to love herself before she can make you feel wanted and needed both intimately and mentally. She isn't mentally able to give you what you want and a counselor can help her to learn to love herself again and appreciate the positive things in her life.
Just tell her you are worried about and have been for a while and that if you didn't love her you wouldn't care about her issues but you do love her and want her to be able to live life to the fullest and you don't feel she has gotten over what her step father has done to her and then suggest counseling and maybe volunteer to go with her for support but if she does agree to it and wants to do it by herself allow her that because she will be no good to you or anyone else if she doesn't get herself right first. Be totally supportive and loving during this time.