ya the rules of it are weird but I see a few couples who pull it off. Hannah and Ben, Francesca and Mark. those stick out anyways I think the rule is usually no off camera fucking without the other person present and no kissing.
I can't see me handeling it, but i think love and sex are completly diffrent and some people can keep em seperate enough to maintain a relationship. I have had threesomes before with men I "loved" at the time and I found watching them fuck very erotic.