Sure. They want to give you the choice of who to romance. Of course not every member of the crew is bisexual, but if I'm playing Fem Shep (as I tend to do because the voice acting is fantastic), I want to romance the female characters because IRL I'm a straight dude. The conceit of having pretty much every romanceable female in the game be bisexual allows me to do that.
Of course it's not realistic, but neither is just saying the right things, checking the boxes until a character inevitably falls in love with you.