Too Much Pressure
For many women, getting married and having children are absolute musts. They feel a lot of pressure -- from society and their families -- to get married by a certain age and start producing the next generation. While we're long past the era of "old maids," and many women are choosing to marry and start families later in life, that pressure never goes away. There's also the matter of the biological clock. Men can theoretically father children at any age, but women generally experience a little something known as menopause that eventually renders them unable to bear children.
Men don't typically feel the same kinds of pressures that women do when it comes to marriage. There's also still a lot of glorification of the bachelor life and the freedom that it entails. So it makes sense that even in a long-term relationship, the man may not feel as strong of a desire to make a legal, lifetime commitment. In his mind, what's the rush? However, some men agree to get married because their girlfriends (or family -- his or hers) have pressured them. They want to end the constant conversations, and they want to make their girlfriends happy, so they go for it. But when they see their girlfriends go into wedding-planning mode and start thinking about the actual marriage, they realize that they only got engaged due to pressure.