Men love breasts and their only mantra is ‘the bigger the better’, so don’t get low on yourself; its normal to desire for a woman with bigger breasts but underneath all the physical attributes, I guess you both love each other for what you guys are, so I would ask you to settle down with her rather than looking at her flat chest.
Just because she doesn’t sport a big rack doesn’t mean she is flat chested; some women bloom late, so this girl might start developing breasts post marriage or while lactating. As much as I love seeing a woman with a fine rack, I get excited with a woman who has small breasts. Its actually about the figure you should concentrate on.
Have no doubts about your future sex life. Sex is called love making because you get to express your love through this way, so it doesn’t matter how big you’re and how great figure she has. Good luck with your life mate.
As you rightly mentioned, you’d look like a complete jerk if you ask her breast size, but I guess you can guess for yourself her approximate size by watching her up close without noticing you stare at her breasts. Since you did mention that she showed some cleavage, I guess she is not actually flat chested.
Your fears are absolutely imaginary; there is nothing like that. 1st of all you’re not into her for sex; you have feelings for her and love her, so I don’t think her physical appearance would be an issue.
We desire what we don’t have, so its normal that you compare your girlfriend’s breasts with other women’s breasts; slowly you’d get over this fixation.