Well, it all depends on what you mean by 'love'. I totally agree that sex should be confined to a relationship and would suggest that a month or so is a decent enough time to wait before sex should enter a relationship. But to wait until you are in love with someone is perhaps a little strong. It can take months or even years before you realise you love someone-some would even argue that you can't know you love somone until you have shared the special bond that sex brings.
Also, I wish to suggest that your dad is right but I believe he means it in a more innocent way then I think you have interpreted it. As I see it, you wouldn't want a relationship with someone you don't fancy would you? So in some respects, no one enters a relationship without sex at least being in the back of their mind.