I always believed that she married him because he was so rich and she thought he could protect her and her kids. By marrying him, she got the kids out of the country and away from all the other Kennedy kids.
I remember reading that on the night Bobby was shot, in the waiting room, she was screaming "They're killing Kennedys and my children may be next."
I have no doubt she married him to get out of this country.
Now that you mention it, I'd heard rumblings about that myself...that she didn't really like the Kennedy clan as a whole and didn't want her kids hanging out with them. Can't say she succeeded with JFK Jr., but she did a bang-up job with Caroline.