Is it gay if you keep getting an erection when you are with one of your mates in particular?

You have to decide using other things. It's possible that erections just happen spontaneously.

Do you think about him in your sexual fantasies? Or do you think about the opposite sex? You are gay if you are romantically or sexually attracted to the same sex. Only you can tell if that's you.

If you are gay, it's no big deal. There are millions of us. It just takes a bit of time to get used to it when you first discover it.