living bicariously
Experiencing intimacy with a person of the same sex through someone else. Note that the person whom you are living bicariously through can be of either sex.
Bob: Hey John, why are you so close to Tina since she's been dating Rob? She barely even has time for you since she and Rob bang all the time.
John: Believe it or not, I find myself really attracted to Bob, and I'm living bicariously through Tina.
Bob: Bro....