Galen's enemies drug him, in a sexy way.
After everyone but himself and Jyn died on Scarif, Bodhi did his best to hide from anything resembling excitement. Unfortunately, the Force - and the New Republic - had other plans.
Bodhi lied to Cassian about how he and Galen met. Obviously the actual truth is "space Grindr."
Before Galen's message made it to Saw Gerrera, he had to find someone to take it. He saw Bodhi as a relatively easy mark. It might have been easier for them both if it had stayed that simple.