"Sow to the flesh" means you do what your body wants, like going with men, getting drunk, etc.
At the moment we think it's nice, but God has given rules, because it's not always good for us.
"Sow to the Spirit" means you do something that builds up the Church, like encouraging other christians, or praying, etc.
Sowing in the flesh is very unhealthy for a congregation.
For example, I've heard about a pastor who asked in church if people could help a brother who wanted to leave his wife for another sister to move...
And my mama left church with us many years ago because of all the hypocrite behaviour and gossip.
We're not allowed to hurt brothers and sisters, so even making them leave church is very bad.