I'm thinking of setting clear boundaries with a male friend
It sounds like it would be hard to set boundaries with your friend
Oh, no it's not. I've been setting clear boundaries with male friends I've known since high school.
If you want to maintain close friendships with the opposite sex, then it can be responsible to make your expectations known up front. If you have been doing this nicely and without causing offense, then you should keep doing so in your future friendships.
I just don't want them to expect me to be their girlfriend one day.
That's a perfectly legitimate reason to protect yourself. Your feelings matter, and no one should try to pressure you into doing things you don't want. Keep standing your ground and they will listen.