I think it has to do with how we associate power dynamics with body parts, as well as the integration of justice to a wrong-doing. The penis as a social/cultural symbol in today's society is one of power (as well as masculinity, which is really just a sub-sect of our ideas about power). In cases of a man cheating, his penis is not only a symbol of his power in the social world, but now his power over her and other women in the sexual world, more specifically, an abuse of that power. So then her mind goes to how she could go about regaining her power, righting an injustice, if you will. Then it occurs to her, remove his source of power. It's a twisted thought process, but if you analyze it from a socio-cultural perspective, it doesn't seem so incredibly out of left field.

Meanwhile, breasts are hardly regarded as a symbol of power--they only give women power in terms of being objectified--they are rarely recognized for the awesome, weird purpose they exist. I would argue women are far more likely to internalize their breasts for the biological purpose they exist, rather than the cultural value they hold. So a man cutting off a woman's breasts becomes more like mutilation for the sake of mutilation, because the cultural importance of the organs (breasts vs. penis) are vastly different.

Naturally, cutting off either would be an absolutely mental, disgusting thing to do to a person regardless of how they have wronged you, but it's not difficult to understand how people are internalizing and then reacting (or threatening to react) if you really examine it. Once again, definitely not defending the wife's threat, nor suggesting that your friend's boob-slice threat is any more heinous, just that from the average (read: not all) woman's viewpoint, cutting off a dick is very different from cutting off breasts.