I'll allow it. In the beginning of overwatch the two character's strong sense of duty made them understand each other better than anyone else on the team and they got close. Professionally of course, but also as friends. This one time when they were on a mission, just the two of them, they had a lot of downtime between the action and they bonded even more. At some point the discussion shifted to their past lovers and how to deal with being apart from them while on long missions. That's when Morrison revealed that he has never had such problems since he had never had a lover, dedicating his life to the cause instead. Ana, a dedicated soldier like Morrison but also older and wiser remarked how one should not live without at least once experiencing passion and love.
The rest is history.
Mercy has her time with many of the team members but never stays with anyone. She's just wired that way. She can never truly surrender herself to anyone.
