How does a guy tactfully suggest getting sweaty with another girl to his girlfriend or wife?

Ian Kerner: Open the conversation with a discussion of fantasy. Explore your turn-ons, and then ease into the fact that you want to have a threesome.

Your honesty about why a threesome is so appealing will make the conversation go better and bring you and your partner closer. For example, explain to her why it would turn you on to see your partner with another woman. Make sure it's clear that this is about her, and not just fooling around with a different woman.