Manfriend

A guy who one is seeing, but is not in an "official" relationship with.

This may imply that one may be seeing other people at the same time, or may be working towards a relationship status with said manfriend.

See also:

Fuck buddy

Potential boyfriend

Person 1: "Oh so are you guys dating?"
Person 2: "No, he's just my manfriend."

OR
Person 1: "Oh so are you guys dating?"
Person 2: "No not yet, he's just my manfriend right now."