I think for me, emotional cheating would be worse. Now, bear in mind I'm being hypothetical since I'm poly, but I think sex can be just sex, I can see that, but love without sex isn't that different from love with sex to me. It's the same thing. While sex without love and sex with love are two different beasts.
I'm not sure if I'm making sense here... I think feelings for someone aren't weaker or stronger depending on whether you have sex. I think intimacy is important, but it's possible to achieve in ways other than sex. However, there is sport sex, that's purely satisfying a biological need (or "want", I guess), and lovemaking, that is entirely different.
So for me, if you're in a committed relationship and you have casual sex with other people, it's not the same kind of sex than the one you have with your spouse. So I guess it's less threatening? While if you're in love with someone else, it's the same kind of love. So there might be a worry that it will replace the love they had for you, while with sex it's not replacing, it's complimentary since it's a different kind of beast.
EDIT: funnily enough, as I first approached polyamory I had no problem with the idea of my husband being in love with someone else: I knew how much he loves me and was comfortable and confident that this wouldn't change. But I had a small phase in which I was scared about the sex potential. He had never been with anyone before me, so I ended up figuring out that there were two things: 1) I would lost my "status" as his only partner ever. I realised pretty fast that it was a rather silly thing to be possessive about so I got over it quickly. 2) There was the fear that, having known only me, he didn't realise I was terrible, not fulfilling his sexual needs, etc, etc, and that anyone he had sex with would be so much better than me that he'd love me less as a result.
I got over that rather fast too, once I understood what it was I had a problem with.
Last edited by Tonberry; 10-17-2010 at 02:35 AM.