I actually have been offered sex with no strings attached, but I have a gf, so the answer is always "No." It's not that I'm not tempted, but I couldn't live with myself if I did. It's not even if she might find out, because in this scenario you said that she wouldn't. I would know and that's enough to make my conscience eat me alive. I couldn't do that to my gf, even if the hottest woman on Earth came up to me, I'd still say no. I love my gf, there's no way I could do that to her.