If two are in a relationship (FF or MF) make sure that they each are down with it on a personal level and they completely trust each other. If one is only doing it to please the other or to save the relationship/marriage, the 3rd party could get caught up in some drama - either somebody may have trouble getting into it or they could harbor feelings for the 3rd party later or be accused of it later.
Basically, if you're trying to convince your girl to do it, don't push her. If she's not really down, it could do more harm than good if she resents you for it later. If she shows any interest at all, you could start with porn (3somes, lesbian porn) to see how she reacts to it. The strip club may or may not turn her on...just depends. Talk about it.