Update:
The s2Member team has resolved this bug and it works perfectly.
Currently there is an incompatibility with s2Member Pro and BP Disable Activation.
The BP Disable Activation plugin is not required as s2Member Pro has a tool to allow users to create their own password during registration. When that is toggled to YES, users are able to immediately log in.
s2Member and it’s Pro version is an excellent plugin, it has excellent support, and I highly recommend s2Member Pro.