The key word there is "fantasy." Fetishes are at least 90% mental. Take away the fantasy aspect and it is really just a woman who is having sex with two guys, one of which is more likely to get her pregnant. Technically, there is nothing about this fetish that requires sex with no birth control. So this makes it no different, in the physical sense, than a vanilla open marriage.
Are we talking about the OP’s specific situation, or the prevalence of this breeding selection within the cuck culture, one of which is
likely to get her pregnant?
If he/they are denied all access, meaning no penis (specifically that penis) is to get near this vagina, or they have his/their dicks locked up in cages, or some clever device in, yes, I think it's likely only one will get her pregnant.
Say I have a submissive female partner. We engage in BDSM as a "lifestyle." We decide to have a child. The fantasy is I, as her Master, am tying her up and having my way with her in order to impregnate her. She becomes pregnant as a result. Are you going to say the child is going to be somehow damaged by our role play? I don't foresee a time where we would sit our kid down and say, "Son, we have something to tell you..." I mean, is anyone really going to argue that the child didn't consent to being the product of kinky sex?
I think you missed the point. In your example, you decided I’m assuming other reasons to have a child, not as an extension of your kink.
Let's modify your example. You have a slave or submissive who happens to be married with 2 kids. Let’s say a boy about 8-9 and girl who’s 13-14. You and your slave have a very vigorous intense online communication most days, discussing very graphic scenes using the most vulgar language possible. In these emails, you discuss training protocols and practices, and specific bondage techniques you have planned to "rape all of her holes." Butt-plug training so you can anally rape your slave on your next encounter. You get the idea. Lots emails covering 6-8 months, with lots and lots and LOTS of specific graphic content.
OP's slave becomes pregnant accidentally. (wink wink) Everyone’s quite puzzled.
One day, after her daughter left for soccer practice, and her son was at a friend's, she decided to quick get her email from you. The daughter had been on the computer earlier for a school assignment. Instead of logging out, and logging back in under her own username, she just popped on the her Gmail account to get the latest instruction or whatever.
Somewhere in the reading or reply, the doorbell rings. She thinks it's the UPS guy, or the dry cleaners dropping off clothes. A moment's interruption, at best. Not so. It's the neighbor dropping by to return some borrowed stuff she used for a party. The gab fest starts, and before you know it, the family is now home. Dinner, etc.
Later, the daughter goes back to do her homework and reads how there's a very good chance her new little brother is the product of rape. And rape by a person she doesn’t really know, hasn’t really met. She see the pictures and video clips of the proposed scenes brought into reality matching not just a word, but very specific words, with unmistakable images of her mother.
Is perception reality? How long will that secret be kept, if you think it should be kept?