I have a 71 60hp motor and I suspect it is probably identical to yours.
When you pull up on the warm-up lever it moves the throttle cable which is activating the safety switch. According to my manual, it should turn over in gear at low throttle. Your safety switch is probably out of adjustment. I'm having that issue with my motor now, it stopped cranking over and I've traced it back to the safety switch circuit. I've not pulled the flywheel yet so I'm not sure if it's bad or just out of adjustment.