Show your dad this.
"Ports" are just like doors. People have to open doors to walk through them, right? Same with routers and some programs, like Blockland. It needs to walk through the door to get to other places. Opening that door for Blockland or other programs is called "port forwarding."
Routers don't have anything to do with your computer, they just forward the Internet to your computer. A router can not receive a virus, therefore the virus cannot be transferred to a computer.
There are always tons of ports on your router forwarded by default internally, and your computer still isn't getting plugged with viruses, right? And the people running hundreds of these Blockland servers forwarded and aren't getting viruses through this, right? So, there you have it.
Viruses aren't even sent through ports. Viruses are sent through Internet scripts, webpage downloads, and some files available for download. Blockland, the ports it requires, and your router don't have anything to do with them.
Of course, my router won't even let me forward ports to a static IP, so I can't do it. Don't ever buy a Gigaset router. They suck richard.
Sources? I've been working with computers since I was 3. Ask me nearly anything.
If you need to consult further, contact Badspot. He's the sole creator of Blockland and he knows more about this stuff than any of us.