Because he was following the map he found in the church that Abe had given to Rick indicating Abe's group would be heading to DC, and the map had the words "Rick Grimes" on it, so Morgan figured Rick might be heading that way, too.

 

If the women refused to marry him, they would be just be slaves for the Saviors.

Giving someone half of everything you own, make and find (sometimes more than half) for the illusion of being protected and cared for is slavery. Especially when you consider that the consequences for breaking the rules range from mutilation to torture to execution.

 

He was following Rick because they knew each other and he wanted to meet up with him again.

 

The Saviors were the ones who took out that group in the forest with the barbed wire. Really I promise