Why would my babies father do this?

Well it's not something that you can just tell someone and expect them not to be in shock. He probably ran away because he was scared now he's trying to make up for it and maybe the conversation didn't go exactly as you said it did. Maybe by what you said he thought that you didn't want him in either of your lives. Even if you didn't say it it's possible it came acrossed that way. It's also possible his parents told him he should try and work it out with you. As for you friends telling you he's sleeping with alot of other girls. Well he may or may not but teens even young adults talk and not everything they say is true. In my opinion if he got you pregnant he more than likely would go sleep with alot of people because he'd be worried it may happen again. Don't believe everything you hear.