A bi person in a relationship with a masculine partner - as I am - might get labeled as straight unless we signify otherwise similarly, a bi person in a relationship with a feminine partner might get labeled as a lesbian. There's no 'one' way to dress bi, and it's not as clearly defined as, say, butch and stud lesbians. It's both the subject of light ribbing, thanks to ever-present meme culture - you love hoodies, flannel, or leather, always expose your ankles, and cuff your jeans and tuck in your shirts all of them except the flannel apply to me - and a way to de-erase ourselves from the queer community. Bisexual style, then, is an interesting kind of intersection.
Intentional dressing can be a powerful form of expression and an important signifier.