The gexiv2 port intentionally allows one python2x and one python3x variant to be selected. However the boost port intentionally allows only one python variant to be selected regardless of version.
MacPorts passes variant selections (apparently regardless of how the variant selection was supplied—by you on the command line, or in your variants.conf, or by a port in its default_variants) down to any dependencies that are not yet installed.
Clean boost and install it with only one python variant of your choice.