Moral arguments aside, I don't think it is right to judge someone and say that threesomes are just wrong so you shouldn't do it.
I think it depends on how both members of the couple view sex. If you view it as a means to pleasure, and you are absolutely sure that both people in the relationship view sex this way, then what's wrong with having a threesome?
However, if one partner views sex as an emotional connection and not simply physical pleasure, the thought of the other wanting a threesome could seriously hurt that person.
I don't think there's anything wrong with it, but I think it is very very important to be absolutely sure that both partners feel good about it before going into it and that both partners share the same attitude toward sex. And also, it is important for a partner not to feel pressured into something they aren't comfortable with just because they want to please the other person. A relationship is about two people caring for each other, not just making sure they get what they want. And I totally agree that there are millions of ways to make sex exciting between two people. Toys, videos, food, games, new positions every month, role playing, whatever helps.
Just my opinion.