I think you're confusing the term ship with OTP (not that OTP isn't often confused).
You can ship as many pairings as you want. Shipping someone just means that you enjoy that pairing. In fact some people ship open endedly. It wouldn't be wrong for me to say I ship Rodney McKay/Anyone (different fandom, but actually true in my case). I love that character so much that I'll read him in any situation, no matter the pairing.
An OTP, or one true pairing, is a different matter; you shouldn't really have more than one OTP (at least not per fandom), some people don't have any OTP. But an OTP is what you beleive in more than others, or would defend to the death or whatever.
I'll just say I also don't know how somebody could get through the whole show only supporting B/M I know! One of my sisters friends only like B/M and *hates* Justin. I don't know how they get any sort of enjoyment out of the show.