1) Men marry women they don't particularly like and don't even find physically attractive in order to avoid being the only guy in the neighborhood who doesn't have a sex life. In America, celibacy isn't just physically uncomfortable. Men who've reached their 20's and are still virgins or suspected of being virgins are treated in an increasingly unpleasant manner by their peers. If you've ever wondered how women who look like Honey Boo-Boo's mother manage to get pregnant, here's your answer. Men who fear being outcasts will screw anything...just to avoid being teased, ostracized, or otherwise punished for not screwing something.
2) Men marry very attractive women they have no respect for, don't like, and frequently abuse for the same reason rich guys buy expensive sports cars. Nyah! Nyah! See what I have? You wish you had one of these don't you. You're not man enough to get one of these. Nyah! Nyah! What could possibly cause such a relationship of equals to fall apart? Yes...that was sarcasm.
3) Oops! I'm pregnant. Shotgun weddings are like leaking fuel tanks. It's gonna blow sooner or later.
4) Believe it or not there are still a lot of women around who won't have intercourse with their boyfriends before they get married. This used to be the normal way to do things. Today it's almost as much of a disaster waiting to happen as a shotgun wedding.
There's my short list. Feel free to add to it.