Not necessarily. If the rest of her is to my liking (petite, dark-haired, fine-boned face, maybe some hair down there) then I really don't mind. If I care about someone I don't really mind too much if she has large or small (or almost no) breasts - I like the -person-. Larger or bigger breasts help, but it is not a deal breaker by any means. My own opinion, but it often seems from porn pics I've viewed that smaller-breasted girls have puffier or nicer vulvas anyway...
My wife is a B cup, so I am definitely attracted to a wide range of sizes. however, my ideal size is like a 36C or thereabouts. I don't even mind fake (I reached this conclusion after my wife suggested a boob job for her own self-confidence; I of course told her she didn't need it... but part of me hopes she will. So long as they are firm, round, perky, and bounce, both while walking and while fucking, I am a very happy man.
There are so many reasons to like a woman. Some physical some not. Breasts are just one aspect of a woman. I certainly wouldn't choose to be with someone because of the size of their breasts, big or small. I've found flat chested women very attractive in the past.
Lucky for me my husband is an ass man, I am very small woman and almost no tits but that doesn't seem to stop my husband. Like I said I think breast size is over rated just like comparing cock size. Everybody is different and that's the fun of it all.