How old are you? If you are super young, it may be that your chest will develop in time (along with the appropriateness of having sex). But if you are an adult, you will probably find that guys who are attracted to sleek, slender women will find you appealing. Guys who like that look do not expect huge boobs.
Do you wear a really believable heavily padded bra? If not, any man you have sex with will already know about how big your breasts are before you take your clothes off. Plus, usually you do cuddling/touching behaviors in a relationship for awhile while you're dating, so by the time you're ready to actually have sex, he'll know what your chest looks like.