While having a rule that a third person can only have sex with the other two *at the same time* (i.e. the threesome rule) does sound unfair to the third person, I don't see it as wrong or controlling... IF the third person is made aware of the rule from the start and agrees to it.
If someone enters into a relationship with eyes wide open, fully aware of the rules and agreements, they are accepting and agreeing to follow those rules. If they know they can only be sexual with the couple together and choose to enter that relationship anyway, they have chosen to only have threesome sex, they *know* they have only chosen to have threesome sex, and that's their choice.
And as with anything, if they decide it isn't working for them, they can walk away.
To me, the threesome rule would only be "reprehensible", as nycindie put it, if the third person wasn't told about it until *after* he/she had gotten involved with one or both members of the couple.
It definitely isn't something that work for me personally. For one thing, I have no interest whatsoever in women. I wouldn't become involved with a couple under those circumstances, and I wouldn't ask anyone to become involved with Hubby and me as a couple. But that's *my* choice. Other people's choices are up to them, at least among consenting adults.
I do agree about the "loving all parts" thing, though. Family is a perfect example. By that logic, I should love and accept Hubby's aunt, even though she spent a Thanksgiving dinner ranting at me about being a shitty parent and calling Country names, because the conversation at the dinner table got too loud and Country couldn't tolerate it, so she put her hands over her ears and said, "Everyone please be quiet!" And then got up and left the table. The aunt found that proof that I can't parent my children and that Country is "mentally defective"...and the worst part was that not only did the aunt know Country is on the autism spectrum--her job involves WORKING WITH KIDS LIKE COUNTRY. We haven't spoken to her since, and everyone in the family knows that if she and we are invited to the same family event, we won't be there. Most of the family has chosen to continue inviting us and leave her out--even her own daughter.
I don't think *meeting* everyone in your partner's life is necessary, let alone *loving* them. And that even goes for metamours and the partner's kids, if any. I chose to ask Hubby and S2 to meet; they chose to agree. S2 chose to let me meet Beads and Spikes and I chose to do so; I chose to let him meet Country and Alt and he chose to do so. Meeting anyone in your partner's life should be a choice, not a requirement. (Personally, I would make an exception about meeting metamours where kids are involved, though... For example, I'm kind of surprised S2's ex hasn't asked to meet me yet, because she knows I'm around the boys sometimes. When my ex met his current wife, and when I met Hubby, my ex and I said the other's partner *had* to meet us, because we each wanted to know the other adult who was going to be around our children. But that doesn't mean I'm friends with his wife, or that he's friends with Hubby. Nor would I want to be friends with either of them; I can only tolerate my ex in very small doses.)