Because scullers row with two oars all the time, they generally tend to have better technique, and are more inclined to be bisweptual should they ever switch over to sweeps.
It's cleverly worded to sound like "bisexual", so those uninitiated to rowing will think you're mysterious and rebellious.
As far as being a hackneyed expression, it's the equivalent of saying that you're bi-sacksual.
Tom: Sure coach, I'll do it, after all, I'm bisweptual.
Coach: Ah, I see, so you could say that you row both ways?
Tom:...that's right Coach, anything for you.
Coach: Alright, well today you're going to be stroking it also. I expect we're up against some stiff competition.
Tom: Well I'm sure we can beat them off, if not, I'll just jack up the stroke rate.
Coach: You're sure you can handle that?
Tom: Hell, they'll be so scared of me, they won't even come.
Coach: Ok, this is getting too gay.