Forwarding a port is literally opening your network. It doesn’t matter what the reason for doing so is, whether it’s torrenting or gaming. That’s why it’s called both “forwarding” or “opening” a port depending on who you ask.
It’s a small risk, but it’s a risk. If you open a device on your network to the Internet through a forwarded port, there’s a real possibility that somebody will find you and attack you. There are bots that spend their entire life looking for common open ports on random IP addresses. And the ports used for torrenting are probably some of the most common. Best security practice for any network is NOT to open a port, but rather to find an alternative method of accessing things outside your network, like a VPN or reverse proxy. In the case that you do need to open a port, that device should be isolated on your network, preferably in its own VLAN.