The short answer to your question is that teenage boys (and men who have not mentally outgrown the teenage mentality) like big breasts and don't care if everything about them screams, "Fake!" Once you're dealing with men who act like adults, we generally are a little bit turned off by obvious implants, and prefer the real thing. We want them to look and feel natural, and to match the rest of the woman's body. Small breasts look just fine on a slim woman's body, whereas a pair of gravity-defying DD's can look freakish. Other women also can get extremely nasty and catty towards a woman with breast implants. You can have the surgery if you want, but it's really not necessary, and I wouldn't recommend it except in cases of reconstructive surgery after a mastectomy or a disfiguring injury.
As for your negative body image and mindset, let me speak to you as if you were the daughter I never had (my wife and I have only sons), but who'd be about your age right now if I did.
Losing your virginity is not a race, and still having yours at age 20 is not good reason to come to the ridiculous conclusion that you can't sexually attract men because of your flat chest. Your body and looks ARE one of the first things a total stranger will notice (they're ALL he knows about you at that point), but as he gets to know the real you (like after about 5-10 minutes), looks rapidly decrease in importance relative to who you are - your personality, attitude, beliefs, confidance, disposition, likes and dislikes, etc. It really IS what's on the inside that counts. Any man who can't get past what's in your bra to care about what's in your head and your heart isn't a man worth having long-term anyway.
The number one thing a man wants from a woman in bed is enthusiasm. If you're eagerly naked in his bed, are making clear that this is precisely where you want to be, and that you think making love to him is going to be wonderful, he's not thinking, "Her tits are tiny." He's thinking he's the luckiest man alive, and is thanking God you're his. You've got Kate Upton beat, hands down. If seeing him makes your day, you smile when you see him (BECAUSE it's him), and are with him because he's your choice, you make his day, and he'll go through Hell and back for you.
My advice is to socialize and hang out with good people who share your interests and values. Eventually, you will meet a young man who cares about you and makes you feel special, who feels special to you, and with whom the thought of building a life together is appealing. When you give yourself to that man, the fact that you're having sex WITH HIM will be orders of magnitude more important than the mere fact that you're having sex. Likewise, the important thing to him will be that he's making love WITH YOU.
Good luck. You have my best wishes when you find that special man for you - it's worth it.
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I'm now 29 but had this same issue/fear when I was 20. Eventually you meet a dude who tells you you're being ridiculous, makes you feel totally comfortable, and you realize that a small chest is fine. If a dude likes you for your chest that really isn't what you should want anyway. I promise you that in 3 or 4 years you will feel ridiculous for feeling that way even if it's not what you think now. You'll be glad that your boobs aren't sagging, you dont have stretch marks, that you can workout without worrying about them, and that you get attention from men for the right reasons.
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I know this thread is almost a year old now, but in hopes that the women involved come across it again, or any other women struggling for that matter, I will put my opinion in. From a healthy, objective and experienced man:
You may find it hard to believe, but many men prefer small-breasted women. I myself am married to a wonderful woman with a smaller cup size(AA, for that matter). I find women of all ages, shapes and breast sizes can be attractive. For 99.9% of women(especially ones with small breasts), the thing stopping them from getting dates/men is their very own opinion of themselves. I bet so many men have looked at you with wanting eyes, but you were so down on yourself you thought they were looks of judgement instead. Of course, I can't say with 100% accuracy because I haven't seen you, but I'm willing to bet you fall into that category.
I've always found women with small breasts more feminine. I find women with large breasts more masculine, and it's actually rare to find a woman with larger breasts who is actually very feminine. The day you accept yourself is the day of your freedom, and will also be the day you can accept a man's interest properly.
I've known many women in my time, who when we get talking seriously about what they think of themselves, or what they think men think of them, it's downright negative. These women are delusional - they believe men judge them poorly based on one physical feature ... most of the time that feature is actually attractive!
Get a hold of yourself, look into Hollywood and see the best looking women there. Keira Knightley, Michelle Pfeiffer, Olivia Wilde, Claire Danes, Maggie Q and many others, are the most beautiful women in Hollywood. They all have small breasts. Do the math.
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