The dishonesty factor is a big one, and not to be minimized but I agree that dishonesty in other areas of the relationship doesn't necessarily signal the end of it, where in our culture, dishonesty about sexual fidelity does - which is stupid and need not be the case. The optimal thing is certainly to agree to some form of honest and ethical non-monogamy, but it's really a function of a patriarchal culture that ownership of your partner is considered to be a reasonable and normal feature of commitment.