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Have her think using a big dildo to stimulate her during foreplay, make sure it's warm and has plenty of lub. Tell her you want to spice things up and seeing her take a big dildo would really turn you on. If she agrees wait until she's really aroused then introduce the dildo while giving her oral sex, this is a powerful combination that will rock her world. If she enjoys the dildo play then you can approach her to think about how great a real cock will feel and how exciting it would be to fuck a well hung stranger. Role play to stimulate her imagination, the brain is the most powerful sex tool.You got to make it fun and entertaining to wet her desires, keep her thinking about it. It may not seem to be working but give her time to fantasize and consider the pleasure and excitement she could be enjoying.
 
I've dreamed about my wife fucking a well hung man for sometime now, she was pretty vanilla when it came to sex. I introduced her to big dildos to get her thinking about how good a big cock would feel as according to her she never had one, it worked, she goes crazy when her pussy is filled. After she had time to really get into the dildo pleasure I asked her if she ever thought about having sex for real with a well hung man. She said she has thought about it but it was just a fantasy, I told her that I would love to see her having sex with another man, it would be such a turn on for both of us. I don't think she believed what she was hearing, I told her I was serious. She asked why would I want such a thing. I told her its been a fantasy of mine for quite a while and I hope that she gives it some serious thought. I could see she was considering my request which I was happy with as she didn't flat out refuse the idea. She said she needed time to think. A few days later she started asking questions, would I get jealous, what if she started getting feelings for another man, what if she liked his cock better. I told her I would not get jealous but excited, she would only have sex with strangers we both agreed upon and no sex repeatedly with the same guy no matter how good a fuck he was.I was to be present at all times both for her protection and my satisfaction of watching her perform.If she met someone she wanted while she was out then she could bring him home so I could watch. I told her that I wanted to film and take pictures so we could enjoy watching her in action later and let her relive the passion and excitement.I told her it didn't matter if she wanted a black man just as long as he was hung. I could tell this was getting her excited, Now I just have to be patent and let her imagination go to work. It will probably be some time before she decides to go for it, but I have faith its going to happen.