Husband sexually stimulated by wifes infidelity...

For years I have been excited by the thoughts of my wife making love to another man. The idea is exciting to her also and over the last few years she has been having an affair with a guy she met at work. She meets him for sex every few weeks. In the day or so before her meeting I get very turned on imagining the two of them together and she will tell me things she is planning to do with him, clothes she will wear, ect. Our lovemaking is intense at this time. After her meeting with him I can't wait to be with her again. She describes in detail what occured during her date and we make passioate love. Any insights as to the reasons this sort of thing would excite us so? Thanks.