At the party she cries: "Never marry a Wop! they treat their wives like shit!"
For GF and GF2 this seems to be true:
Michael lies to Kay, leaves her alone, treats her like his property.
Sonny betrays his wife openly.
Carlo beats Connie.
From the novel we even know about the unfortunate marriage of Johnny Fontane.

There are some exceptions: Vito seems to have treated his wife respectfully, but she was a traditional Sicilian. Not a modern woman who claims some rights. And in the novel we learn that he the reason why he didn't beat his wife was that it wasn't necessary.
The other exception: When Michael teaches Appolonia to drive they seem to really have fun, a playful marriage. Though I think, we know already that even she wouldn't get killed, the marriage would be terrible. Listen to her voice when she niggles: "Michele, me a promiso!" ;-)

But I think that in the beginning, Michael's relationship with Kay was very promising. They were open, playful (the bed scene!), tender. But when Vito's shot, he loses her.