One more in the "do it if you want to, otherwise don't" camp here. Based on your (way too incomplete to be trying to make any sort of "binding judgement" based on it) description, it sounds as though he's more into "watch a woman get fucked by a dog" than actually being zoo - nothing intrinsically WRONG with that, but it can lead to somebody pushing more than he ought to.
From what you've said, I'm guessing he's not interested in "playing bitch" for any of the dogs - but mainly wants to see you doing exactly that. *IF*, and *ONLY* if you're willing to give it a go, you might offer him a "tit-for-tat" proposition: Use prettier wording if/when you spring it on him, but basically, the sentiment should boil down to "I will if you will - but you gotta go first!"
If he flat shuts the idea down, let him know you feel the same, and you expect that to be the end of him bugging you about letting the dogs fuck you. BUT ABSOLUTELY DO NOT USE IT AS A BLUFF! Be ABSOLUTELY CERTAIN you're willing to let one of your boys mate you before you actually make the offer - I *SUSPECT* he'll do the "Uh... no, that's OK, I don't want no dog dick up my ass" thing, but he may be gutsy enough to go for it for the sake of being able to see you getting bred. If that's the case, you'll find yourself on the hook for it, and it may turn out to be the end of the relationship.
With all that in mind, you need to ask yourself whether letting one of your dogs mate you is good, bad, or somewhere in between for you - You say you've collected from them, so you know the physical details of what they'd be putting inside you, and I suspect you're aware that unless you're allergic (you'd probably know it if you are, since you say you've collected them) letting a dog mate you won't have any significant effects. The only worry is about physical compatibility - Obviously, you know about the knot, but do be sure you realize that *VERY* few women can actually "tie" vaginally with a dog - the anatomy isn't right to make it happen - the dog will almost certainly be capable (especially the big breeds you're talking about) of driving himself deep enough into you to form up the knot inside you (and the deeper, the better, or so both my own experience as a male accepting being covered anally, and reports from folks who have a vagina indicate) but it's nearly certain that he'll also be able to pull out of you while his knot is fully inflated, and such a pullout may be momentarily painful, but is very unlikely to cause any actual injury. (an anal pullout, on the other hand, could be quite painful, and actually cause damge) If and when you do make the attempt, be aware that it's an extremely rare dog that can give enough stimulation for a woman to reach orgasm - If you'll pardon a small pun, it's "the nature of the beast" - Unlike a man, he's going to do his best to sink himself as deep inside you as he can, will thrust for a short time, then basically stand there, probably pulling a bit, though it's unlikely it will give you the stimulation you want/need to reach orgasm. How long he stays inside you will be up to the both of you - He's probably going to be pulling by reflex, but that's just him trying to maximize his own stimulation and the amount of semen he leaves in his "mate". It would probably be wise to "follow" him, if you're able to, and definitely if you want to maintain the tie. If you want him out sooner, you can almost certainly pull off of him, but be aware it may take some "Ouch! That smarts!" level pain to get his knot through the constriction at the opening of your vagina.
If you do decide to try for a "premature disconnect", it CAN (not guaranteed will, but can) help to push as though you're trying to "hurry" a pee, or like you need to finish pooping in a hurry for some reason.
Above all, if you go for the idea, remember that once he's inside you, just stay calm, don't panic, and all should be just fine. There's truly very little that can go more than "some pain for a moment" wrong. Just don't expect it to be "world shaking sex" unless you've first worked yourself up to the edge of orgasm before letting him mount.
