Is this from something or are you having wine? Because it's making me laugh either way. I might just start answering my unreasonable relatives this way.
So, now that I am back to my normal self (normal for me, anyway), "Because Jesus" is an answer that is often used when people use the Bible to support ridiculous positions. For instance, when someone says they are against gay people because they are a Christian, the answer can be summed up as "Oh, you've decided to be a moron and a bigot because Jesus". The reason that works is because they are completely misunderstanding their own holy book and religion.
For instance, when Michelle Bachmann, a former congresswoman from Minnesota, says that the president of the US is a secret radical Muslim that is bent on arming Iran with nuclear weapons and destroying Israel and that will usher in the end of days, we can safely say she is saying stupid things and is an idiot, but we say it in a sarcastic manner, like "Oh, well yeah, that makes TOTAL sense, because Jesus", at which point we roll our eyes so hard they detach from our optic nerve and fly out of our skull to escape the stupid our brain just ingested.
Basically, it's used when people invoke being Christian as an excuse for being stupid, bigoted, hateful or someone else unsavory. And it's used snarkily.