First things first, you have to confront you penis insecurity. Penis issues can destroy men's lives. There was a thread on here a few days ago where this woman really liked this guy but he was a total freak about his penis size - she had absolutely no issue with his penis size and said he was great in bed actually but by the time of her third update he had become completely obsessed with penis size and he absolutely destroyed the relationship - and he told her that his last three relationships ended because of the same reason. The guy was losing his mind and destroying his life because he couldn't stop obsessing about his stupid penis.
The fact is you have a 3 inch penis. The fact is there is nothing wrong with that. "penissizepredictor"? Fuck that bullshit. You're born the way you're born and you're going to need to love your body regardless of what you've got. Some people are born deformed, with congenital birth defects, terrible really bad stuff. But for some reason having a small penis can make a man more mentally fucked up than any of that stuff. Because they have unhealthy views of what a penis is "supposed" to be and because there is there very ridiculous view that penis size has something to do with sexual performance.
Dude, get over it. You have the penis you have. Use it. Either your boyfriend will be fine with your penis or he wont - DOESN'T MATTER. Find a guy or girl that likes your penis and live your life happily. Penis size doesn't matter. Love the penis you've got.
The next issue is that "bi is not a real sexual orientation". That's just dumb. Hurtful to you? Yes, I would think so. You may have to find a different boyfriend because your current boyfriend could possibly be an idiot and being 28 years old he might be to late to change.
However, I'm more concerned with you having a healthy perception of yourself than I am with your boyfriend being an idiot.
Love your penis. Tell your boyfriend if he doesn't like your penis to get lost. Tell him if he doesn't think bi is a real sexual orientation he's as much of an idiot as a straight guy saying gay people aren't really gay because god doesn't make gay people, and you won't be in a relationship with someone that idiotic.