Monitoring HTTPS traffic using tcpflow

If you have a copy of the key you can use ssldump which uses a syntax almost identical to tcpdump.

It won't be quite as pretty as tcpflow, but you can get at the encrypted content.

You're missing the point of HTTPS. You cannot read (much) meaningful information from a HTTPS stream, as the TCP layer encapsulating it is encrypted. Ultimately that means that the data will look like garbage to anyone trying to intercept it (including you).