- Leto bought his concubine from the Bene Gesserit. This was considered a normal thing to do, and it is suggested that he wasn't even involved in the purchase; he sent buyers to pick a suitable candidate, and Thufir Hawat ran a background check on the school, which suggests he had the option of buying a non-BG concubine(*)
- Shaddam IV bought? his concubine from the Bene Gesserit. Even though he very much desired to have a son, his concubine bore him only daughters. He seems unable to do something about this, even though the obvious solution would be to get another concubine(**)(***)
- It seems generally known that Irulan is trained by the BG, to a high level even
- I think the later books confirm that training the daughters of noble houses is a service commonly provided by the BG
- We also know that the Bene Gesserit administer a deadly test to their acolytes, or at least the ones deemed worthy of their breeding programme.
- So Irulan was, in all likelyhood, tested with the gom jabbar. What would've happened if she'd been found wanting?
In summary, we get the impression that the upper-class noble Houses seem predisposed to buy their concubines from the BG, even though they know, and have to be aware, that the Bene Gesserit have schemes of their own. They also send their daughters to Bene Gesserit boarding school, even though, um, 'accidents' are known to happen at those schools.
So what power do the BG wield over these houses, that they seem to submit to this situation?
(*) The alternative, of course, is that the Bene Gesserit operate multiple schools for girls
(**) Alternative explanation; the extent of BG control over bodily functions is unknown to modern science, so the series of daughters is presumed to be 'bad luck', completely unrelated to the Emperor's choice in mate.
(***) Alternative explanation II: Shaddam knows that he's walking a political tightrope, and having no male heirs makes him less of a target for assassination.