A man may react to his wife's pregnancy with various feelings such as:
Protectiveness of his wife
Increased joy in their marriage
Pride in his virility (that's one thing men always worry about to some degree)
A certain amount of worry. "Will I be able a good father to this baby?" is a common concern, especially for men who remember their own childhoods as having been difficult.
There can also be, way underneath, a feeling of being left out, which may be expressed as crankiness towards his wife, wanting to spend more evening with his male friends or flirtatiousness with other women.
These reactions are all normal, but some are not helpful to his partner, who needs extra support at this stage of her life. It's very important that fathers-to-be talk about their feelings. When they do, they often find that the negative emotions (fear, jealousy) shift aside, allowing the positive ones (excitement, connection) to come forwards.