Are you lifting for self defense? I don't.
If you were then your son in law would be correct. Is that why you lift?
Your son in law's logic is somewhat flawed because a skinny guy with a knife or gun will kill his badass dojo boy easily.
How about the other guy takes his eyes out? Nasty
A controlled sport fight is different.
He is training his guys for safe, controlled fighting which is understandable.