Literally did it on impulse without even thinking about it so I wouldn't chicken out, so I blurted it out.
I said "I'm bisexual. I like both men and women. No more anti-gay stuff from you, ok?".
He said, "oh go away, you don't know what you like". I think he'll understand eventually, he needs time to think.
EDIT: Showed him this thread.
I said: "Look, this is what I said to you, these are my online friends who are happy for me that I came out as bisexual."
He said: "Oh you don't know what you're talking about, you've never had sex with a man or a woman before, until then you'll never know."
And yeah, I am still holding on to my v-card at 26. I was insecure about that years ago in high school, but I'm honestly glad I've waited to mature first.
Dont ever get into the trap of thinking you need to date say, a man to 'validate' your bi-ness - bisexuals are valid even if they've never been in a romantic or sexual relationship with anyone, you know yourself what you're attracted too.