You have to sit down and think about what's going through your dad's head.
Fathers are competitive with other men, especially regarding kids. They brag about grand kids, colleges, sports, promotions, etc. From what you say, it seems clear your father has this concern. So, what he is really saying may be "it bothers me your so different (from the sons of my friends)"and "Like, nobody's (sons in my social circle are) gay why are you?"
It's rare when a father tells his buddies that understanding and accepting his gay son is the hallmark of a good father....or better yet...when he realizes that being a father means standing with you, and not tossing you aside in his own desire for social acceptance.
So, if this the case, maybe you should provide him with examples of fathers who stood beside their sons in challenging times. One example I can think of is Fred Trump. Back in 1990 when Donald Trump was $900 million negative net worth and imploding...reporters tracked down Fred Trump to get his opinion on his son's huge business failures. Fred Trump's actual statement? "Don't worry about Donald, I've watched him all of his life. Donald is a winner and will have no trouble. Times are tough for everyone, but Donald will be the one who ends up on top."
You can't control what your father will say...but do me a favor, and just imagine you have Fred Trump for a father...and he's saying this about you. So take the statement above, put your name in place of Donald, print it out..and paste it on your bathroom mirror where you can see it. THIS...is the father you deserve...not the one you have.