a transition guy is someone a girl talks to, plays with mentally and/or physically and proposes to be with them until she can move on or find someone better to be with. most common when women are seeking a divorce or separtaion from a past love, or with problems that the woman is experiencing.
ex: Hairy was dating this married girl while she was getting a divorce. Once she recieved her divorce, she dumped him and started dating someone new. He was the transition guy to her.
ex: He was with her while she was going thru problems. He helped her alot and supported her. When her problems were fixed, she left him.