Carmen 91
It's not because I knew you well or thought you faithful,
Gellius, or thought you could keep your mind from vile sin,
that I expected you to be true to me
in this hopeless ruinous love of mine:
but because I was aware that she, for whom a vast desire
consumes me, was no mother or sister of yours.
And though I was closely linked to you by friendship,
I didn't think that was enough excuse for you.
You considered it enough: there's so much pleasure
in every game to you, in which there's any sin.
(Translated by T. Kline at this Catullus website.)