IANAL.

This article convinced me about the conflict: https://people.gnome.org/~markmc/openssl-and-the-gpl.html.

The OpenSSL license imposes certain restrictions on people wishing to distribute their program using OpenSSL, while the GPL-2.0 license says you may not impose any further restrictions when you redistribute the program. There is an exception for OS components, but MacPorts is not part of macOS.