It's because the user to run couchdb is (couchdb) who is not allow to run on port 80. You need to rebuild couchdb and change COUCHDB_USER to root as follows;
1. cd to your couchdb source code directory
$ cd svn/couchdb
2. change COUCHBD_USER to root
$ vi etc/default/couchdb
change COUCHDB_USER=couchdb to COUCHDB_USER=root
3. make and install it again.
$ make && sudo make install
4. then you can change couchdb local.ini configuration file to use port = 80. and restart the couchdb service again.