I'm going to lie to my friend about the gender of my crush.
Do you have a crush on someone?
I'm gay and I like a man who's straight.
Lying to your friend about a relationship that you want to pursue isn't appropriate behavior, even if this is just a crush. Friendships are based on trust. You should be honest with your friend, even if it is uncomfortable, in order to prevent any misunderstandings later.
Maybe you are right
I don't know what your situation is, but at least among your close friends, I think you should be open and proud about your sexuality.  This is who you are, and it's nothing to be ashamed of.  Your friends will accept you, and then it will be okay for you to harmlessly mention a crush you have on an unavailable man.