It's entirely possible. Just refrain from the baggy cargo shorts, wifebeaters and Docs. Lycra is close-fitting and reveals the obvious absence of man-parts, which is, naturally, very feminine. Jerseys are also very flattering if you're so blessed. (and a little if you're not, thank god!)

Femininity itself is subjective. *very* subjective. But there are ladies here with long hair, soft skin and probably totally impeccable nails (Oddly, these subjects don't come up too terribly often...) that are quite the hardcore biker chicas. I suggest a braid or at least a ponytail for your hair, using the nice bands that don't pull and/or break your hair... a light moisturizer with sun screen, and maybe full-finger gloves for your nails? I dunno about that one. I guess it'd depend on how long your nails are. Pack a pair of nitrile exam gloves in your bag for roadside repairs.
If you're riding with your bf and have insecurity issues with your femininity, just look at him, ask if you still look like a woman, all wearing your bike shorts and jersey, with biker calves and shades, and if he says anything to the contrary, he can go date a man.
Many of us manage to do great service to our innate femininity even with short hair and nails, fwiw.