It starts with a kiss.  It's only one, but it's somehow more than that.  They never meant to, not really.  Eric had something he needed, Lucifer could give him something he wanted.  It never should have been more, should have been less, way less, but now they're in each other, now they're so ruefully, woefully stricken that it's almost funny how right their bodies slot together.  Eric on angel blood is far more majestic, and Lucifer on Eric is almost unGodly, but they can't stop, won't stop, and they aren't really sure if they even WANT to stop.