Honestly? I'm a guy and it doesn't matter to me in the slightest. Not in the slightest.
I don't think I represent the majority of men, but I represent a far larger number of men than you probably realise. I've really genuinely never lusted after either breasts or butt. In fact large breasts to me are a complete turn-off, and flat-chests are nice.
I'm physically attracted by a girl's face & hair, but would only even consider dating someone if they were also a really nice person at heart.
So for me a flat chest doesn't make the slightest bit of difference. And I'm certainly not alone in thinking this way.
Honestly don't worry about it. It may even be a blessing in disguise: any boyfriends you make will probably be the best people around and they will almost certainly truly love you for who you are. They'll probably be the best boyfriends you could have. By contrast, if you had large breasts you'd be forever chased by men who don't give a flying **** about you and care only for getting inside your pants.