In what was the motive of the people here?
"And they said, Go to, let us build us a city and a tower, whose top may reach unto heaven; and let us make us a name, lest we be scattered abroad upon the face of the whole earth." Gen 11:4
They failed to do something that is always important... check with God first:
"And the LORD said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do." Gen 11:6
Do we imagine that it will please God to defile our flesh without knowledge that it is His will for us to do so?
What is the will of God?
"And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day." John 6:40
Would we presume to mark our bodies as if we knew it were God's will to do so? Where does God give His approval to such presumption?
What's done is done, but when we already know God and know we need to be following His lead, is it not best to first know which Way He is leading us?
Jesus would have walked on alone if none were willing to do things God's Way:
"From that time many of his disciples went back, and walked no more with him.
Then said Jesus unto the twelve, Will ye also go away?" John 6:66-67
If we doubt our ability to attract people to Jesus by our lives, why do we presume that corrupting the physical body needlessly which God gave us to take care of without His consent will do it better? If you must display something write it on your clothing or on a sign made from paper. Leave the temple of God alone!
We could also choose to go into the nearest barroom and get drunk with ones gathered there in order to make conversation with about God easier, but would that be God's will?