She didn't do it. Tsunade once informed Jiraiya that if she had to pick between him and Orochimaru for a boyfriend, she would surely choose Orochimaru. That didn't necessarily imply she was interested in him romantically. She could have been making an effort to protect his feelings. Also, note that when Tsunade first meets Naruto, she calls him "my boy". This could be taken as a sign of affection towards him. Tsunade is shown to have a strong sense of justice, which makes her unlikely to have any romantic feelings towards someone like Orochimaru. She also seems to dislike the fact that he used her in his experiments, which would make her unwilling to show him any kind of favor too. Now, regarding her comment about choosing between them: It's possible that she was just being polite and not wanting to offend either man by saying something that might cause one of them to leave. She probably wouldn't say this kind of thing normally so we can assume it was just before entering into a very important conversation. Orochimaru was probably aware of this comment and took advantage of it by asking Jiraiya to send him Naruto so he could experiment on him. Tsunade being the kind-hearted woman that she is agreed to let him do so.