If the father doesn't approve of it, and didn't leave the door open for further contact, then that's it.
They are not being disrespectful. Saying "no" should not forcibly come with an explanation, thus you can't demand it nor force them to listen to you. It is called freedom, and that is more sacred than anything you can tell them.
Attending the wedding reception isn't outside the folds of islam however the act itself is outside the folds of Akidah however there's no harm in participating as long as your not consuming Alcohol...or participating in any other acts deemed impermissible.