I don't know how to explain it without sounding stupid... But for me, it just boils down to an overall "pregnancy" kink, regardless if the "mother" is male or female. For me, pregnancy is very beautiful, and can also be very kinky, because of the obvious swell of new life and fertility and parenthood...

And the idea that a man could be so full of life, carry his child (and the child of his lover) inside of him and protect it, despite the laws of nature telling him that he shouldn't be able to do so... It's very beautiful. Throw in a scene where he and his lover are fondly, disbelievingly stroking his swollen belly and I'm done. I'm just DONE. It's too hot.