Honestly, I make it a point not to get involved with partners who have a lot of rules in their primary relationships. Generally, when I become involved with a partner or couple as a secondary relationship to an already existing primary relationship that has a lot of rules in place, I find various situations can come into play:
1) The person or couple is using me to enhance their own relationship with each other. No thanks. I'm not an accessory. I'm a person with needs.
2) The couple is insecure in their relationship which means that I have to constantly worry about inadvertantly hitting boundaries that create tension between all of us. Again, no thanks. I don't enjoy relationships where I have to walk on eggshells.
3) My relationship with that person does not have the ability to develop along it's own natural path because of restrictions placed upon it by other partners.
I understand the need for some couples to put these rules in place, but since it would restrict my freedom within the relationship as a third, it wouldn't be an appealing situation for me.
I honestly prefer to let love develop on it's own and let it grow along the path that's right for that relationship, and I would never want to place any restrictions on how my partners love (aside from safe sex practices for everyone involved). And also, when I'm involved with a couple, I've always found that the more secure they are in their relationship with each other, the more secure I am in my relationship with them.