One never sees a bra or its outline in fashion magazines such as Elle or Vogue unless it is featured as a lingerie item and fully displayed. Bras are never worn in fashion photography.

There are a number of practical problems with bras. Straps or other portions can protrude, giving the image a tacky look. They cause an outline which interferes with the graphics of the outfit. With thin materials under strobes, they may reflect more (or less) light than the model's skin and create a distracting outline in the final image, even though not noticeable under modeling lights. They distort the shape of the bust and leave impressions which may be visible when shooting the next outfit. Anybody considering the model for an assignment and reviewing her portfolio wants to know she looks good without a bra, because she won't be able to wear one if hired.

The outline of a bra suggests:

a) she's overly modest and may be difficult on the set;
b) the bra may be padded;
c) she and her photographer are inexperienced.

Anybody reviewing the photographer's portfolio wants to know the photographer has an eye for detail. Who would hire a model or photographer whose portfolio included images with visible bra lines?

So for this reason, bras are not allowed to be worn on shoots. If this is a problem for you, then modeling is not for you.