Niki - I know many people who have been in open marriages. Sex is a fun thing to do. There's no reason to necessarily say that sex must be about your partner, just as there isn't a reason to say "I will only go skiing with my partner".
I've done months of research into the church and sexuality and it always comes down to children creation. So if you are taking precautions not to have children with someone else, it really shouldn't matter if you choose to have fun with them - whether it's skiing, horseback riding, or fondling