First, I would like to say that it is awesome that you are that in-tune with what turns you on, and open enough with your significant other to express that with her. I agree with Eric in saying that you might want to look into a realistic strapon. That said, I am a woman, and I do realize that there is NOTHING, but NOTHING like the real thing when it comes to a man's penis. If you feel that this is something that you need to fulfill, my advice is that you do not go behind your s/o's back to start a relationship or have a threesome that does not include her. If she is already somewhat jealous or uneasy about the idea, this will definitely make her feel like you do not value her feelings. From your post, however, I can see that you do. Perhaps she does not understand exactly how much this means to you... Keep in mind though, that while she has not exactly been thrilled with the idea, she also has not vetoed it either. I would talk to her about it, tell her the strength of your feelings for her and your (eventual) plan that she should be your wife, and find out exactly what it is about the idea that she is hesitant about. I hope that you are both able to work through it, and that if you do have the threesome it is a stellar experience. Take care and let us know how it goes.
Hugs,
Kate

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I am certain of nothing but the holiness of the heart's affections and the truth of imagination. What the imagination seizes as beauty must be truth - whether it existed before or not.
-John Keats