It's rules one and three that are causing the issues., You need to stop thinking of it as you and your husband having a relationship with her and more of there being three relationships here. You and your husband, you and your girlfriend, and your girlfriend and your husband. He is absolutely right that in a equal triad, the rule would forbid you and him having sex as a couple. If you want an equal triad, you need to consider that. You know there are even triads who never have sex altogether? Only as couples. If you want to use condoms with her, fine, for now, but I hope you don't mean he can't cum in a hole witha condom on. I've come across that rule before, to me it just meant he had to cum all over me bukkake style. I didn't go there though, too many trust issues.
The easiest, most realistic thing to do is abandon that rule, be secure in your relationship with your husband and not to project any issues you guys have onto your girlfriend by having to treat her badly in order to prove your devotion to your spouse.
Last edited by london; 06-19-2013 at 10:23 AM.