The description of female breasts has way too often been abused for male-fan-service. I just won't believe it's there for "realistic" instead of fanservice.
There are only a few times when it is properly used, but descriptions of a character are sometimes necessary - in body-swap stories, for example, or in comedy works. The focus on the breasts (which, for some reason, tend to be likened to foodstuffs) seems only to be there for the amusement of a certain segment of the readership though.