Cristina could be a wife without being a mother. Marriage wasn't her ultimate goal, but she loved the men she was with. She didn't want to be sleeping around.
Owen had a woman who wanted to marry him, and have children, and he slept with Cristina, before breaking up with her. He chose to stay with her, at a different time, and then "punished" her for the abortion, by cheating on her. This whole thing with Teddy, this last season, seemed like their attempt to get people onto Owen's side, because he's been playing musical women for years. He loves Cristina, oh but wait, he might actually love Teddy. No, there's a woman who wants to have children, but wait, here's Cristina, the love of his life. Until Amelia, and then oops, that marriage goes belly-up, but wait, she had a brain tumour, let's try again. Until he flies over to see Teddy, they sleep together, and she throws him out when she finds out that it was Amelia's idea. So he comes back to her, and well, now Teddy is pregnant, and they're happy, until the obsession with Owen is suddenly replaced by a history of cheating, and an affair with what's his name.
Cristina was with Burke, then Owen, and Shane for a little while. She didn't flit between them like a moth between lamps.