This is actually one decision I understand, it's because story wise they're both setup to be heirs to komaris role, if I remember correctly, or something similar, can't remember tobs paragraphs. But basically whoever you impregnate has to step down, so if you did both of them there'd be no heir, komaris already pissed with one so two and she'd probably attempt to geld you. Probably has ramifications in the future of the game or the epilogue.
Why you can't impregnate multiple times though I don't get, especially when miko currently has one of the most difficult cgs in the game to unlock requiring both her and kiyoko to be heavily pregnant, you get one shot a playthrough. Also weird there's no further content with them (at least at the moment), miko seems like a hooker with a heart of gold and mai seems pretty smart and collected, would prefer them over rindo tbh, or at least alongside. Issue is tobs, keros is regarded as one of the gods with the most direct interactions but since he's copying princess maker and wants all the kitsune to be borderline racist Japanese expys it removes a lot of the potential for fun stuff and becomes kinus story while you're a tourist.