It doesn't matter if he's married because at the end of the day, Preston Burke loves her. He will always love her and they both hold a piece of each other's heart. Unlike her relationship with Owen that has to be validated by marriage and/or kids, Burke had to finally let Cristina go when he realized that if he truly loved her, he wouldn't want to force her to do something she doesn't want to do. And now 7 years later, he comes back into her life and it might not be a marriage proposal but it is a fulfillment of a dream he knew Cristina wanted. He saw the potential when she was an intern and he followed her career silently. He gives her an offer of a lifetime. He is a Harper-Avery awardee and a director of his own hospital. He has resources but he wants to do the right thing for his wife and kids. So he does the next best thing, give his hospital to that one person he could trust. Someone who will cherish the gift just like he did. Someone gifted. Someone smart. Someone as ambitious as him. Someone as talented as him. It was Cristina. And it was perfect.