No, I am not a maintainer of the hdf5 port.
It is very difficult to find ports that depend on c++ and parallel because these options are embedded in the hdf5 port.
I’m curious how FreeBSD manages packages when the two options are mutually exclusive.
This is a very undesirable situation. It isn’t handled gracefully. The PARALLEL port option is currently non-default, and it looks like nobody is using it. If some port would need to use the c++ and some other ports would need to use the parallel option - hdf5 should be modified and these options should be made compatible, Otherwise some ports can’t exist.