[Laura-F]

Getting back to the original question; an important thing must be remembered. Your wife/gf is with you at least in part because you make her feel secure. Being less of the man she thinks you are takes away from that security making her feel among other things, less secure. It is important, especially if she is your wife, to maintain her security. As her husband, you are the one that is supposed to sacrifice yourself for her. Make sure she knows that you will always be her man and you should be able to tell her what you need. As far as wearing a bra, I would think it would be a matter of wearing one because you need it to support your breasts rather than just wanting to try it.
Try the pencil test and if it stays, say to your wife/gf, "look at this! I think I need a bra."