As long as the sexual fantasy or fetish dosn't create a "victim", it is all good.
As I see it, there is no victim here. Now if the son's girlfriend, or the son, were to find out, that would be different, but this would still not be the intention of the act.
I expect part of the excitement is the knowledge that it is 'naughty', like masturbating in public. This, as part of the sexual excitement, is perfectly normal.
I am assuming that the father, son and holy panties, along with the girlfriend are over 18 and over 21 in those regions where the code is more strict - to assume otherwise, without strong proof to the contrary, just makes life on this site difficult for everybody.