You are correct. It is possible that any person having sex with another can form a bond, so this is the risk that is taken when anyone moves into this kind of situation. That is what I meant by opening Pandora's box.
From what you have further revealed, that you feel very threatened by the idea that she is having fantasies about other men (even though she denies it), then you are showing some obsessive thought. It may stem from insecurity, but it is a form of obsessive thought. This kind of though is sometimes overcome by compulsive acts. In your case, the compulsive act is to fantasize about your wife having sex with another man. This is a mild case of obsessive compulsive behavior and can be helped with some counseling with Cognitive Behavioral Therapy.
Since you may engage in "partner replacement fantasies", you feel that perhaps your wife does as well. Men probably do this more than women, and the though of this makes you feel a bit threatened or insecure and therefore you have these thoughts about doing the worst to yourself as a preventive, in a sense.
In most likelihood your wife is not interested in sex with anyone but you. If these intrusive thoughts are hurting your marriage or making you very anxious, then I certainly urge you to see a competent therapist who deals with anxiety
disorders. This is not a serious disorder and you can find relief with the proper treatment.
Elliott Sewell, LPCC, NCC, CCMHC
P.S. - research has shown that replacement fantasies are not harmful to the relationship if they ever do occur.