I don't know who came up with this argument, but I thought it was clever:
Are you married? If so, try telling your father-in-law this: "I did not have relations with your daughter until we got engaged." See what he thinks of that. After all, if your father-in-law is as perceptive as you are, there's no way he'll make a spurious assumption about pre-marital relations between the two of you based on such a statement.
It's fun to imagine these kinds of things....This is what it would look like, just for review: In casual conversation with your wife's dad, you mention that you didn't have (relations) with his daughter until you were engaged. After telling him not to read so much into an idiomatic statement and that he should stop being so historically ignorant, tell him your statement had nothing to do with what you guys did after the engagement. See how that flies.