Things are taken slowly and levels of intimacy are built up gradually so that all involved parties maintain a reasonable sense of comfort and security as they get to know one another better. You clearly have a shy dog. The situation is exacerbated by this very fact. People often do not appreciate the fact that dogs can be shy, sensitive, untrusting, etc. To most people a dog is a very exciting stimulus.
Many people get excited in a positive way and want to pet or play with the dog.
Others get excited in a negative way becoming scared and doing everything they can to avoid the dog. Since she is three and you've had her for two and one half years, that leaves six months of early history where she may have had some significant negative experiences with men. The way to handle a situation like this is to accentuate the positive and eliminate the negatives. You know she will go for walks with him. So this is a good starting place for building positive associations.