What reason can I give to an insecure boyfriend?

Well, to some extent he has to deal with it - you'll have male coworkers, neighbours etc.

In some ways your dad is right though. What most women interpret as friendly, men would interpret as flirtatious. The way men interact with women is VERY different from how men treat each other, but not so dissimilar to how women behave with each other.

So I'd guess you may need to be more aware of that, and he needs to build a little confidence you only want him.

Aww. This makes me a bit sad. It reminds me of my life so much. I really don't know what to tell you. I really feel where he's coming from. But my logic is telling me that you should be able to have friends of the opposite sex in a healthy relationship. But my closeness in real life experience tell me to feel for him and take his side. I think you should really look inside yourself and try to find the answer to that question, of why you want male friends... When you find that answer I think you'll find what you need to do.