>I'll invite the monstergirl over
See, that's the kind of practical thinking that I like to see. The only concern I have is that this assumes it's just one monstergirl after him, and that she'll have no competition. And if there's going to be a fight, the last thing you want is for it to happen in your living room. A lamia battling a pack of goblins can cause a lot of property damage.
Now, if you have an especially fearsome monstergirl claiming your man, you might be okay. The others may no to stay away, either from just her scent being around your home, or from prior knowledge that your home and the sexual partners within are her territory.
If that's not the case, you may have to play mediator, as a neutral third party in your boyfriend's sex life, and negotiate when, how long, and in what order the different girls have sexual access to him. I know it seems odd that they'd agree to such things, instead of hoarding him to themselves, but they aren't monsters. Well, they are, but they aren't without reason. Once you explain that they will have fair and equal access to your man, his penis, and his semen, and that he will deposit said semen into their vaginas, cloacas, and other equivalent or desired orifices in due time, without struggle, they'll usually become more cooperative. Just be sure to leave lag time in his schedule - your man is only human (which is what the monstergirls like about him) and does need time to rest and recuperate before he inseminates another monstergirl in your bed.
>You have no idea of the depraved depths of inter-coital snacking, libations, and hydration they will be subjected to
You make a good point about hydration. Make sure you have plenty of slightly chilled water and gatorade in your house. He's going to be losing a lot of fluids with the copious amounts of semen he'll be filling monstergirls with. And he'll need to keep his strength up with healthy, but energy rich snacks.
On the subject of recuperation again, some monstergirls do get impatient and poorly understand the human sexual refractory period. They'll expect him to be erect again in short order, and can be aggressive with him. So you should take advantage of the "Coolidge Effect" and act as a sort of impromptu fluffer for him, making sure he's ready. But just make sure to stick to blowjobs and handjobs and be certain she understands what you're doing. She may get aggressive with you if she thinks you're trying get in between their lovemaking. As long as she knows that you're merely prepping him for her, and you're not trying to do something rude or selfish like introduce your own vagina into the mix, she should be fine with it.