The ideas come from fantasies. The nipples have holes in which express milk, so naturally they can express sh*t too (sh*tting nipples aren't uncommon over at gurochan) and hey..replace the nipples with penis tips and you get sh*tting dick-nipples (also not uncommon at gurochan) and hey...why not shove things up them. I've seen porn where the guys urethra has been penetrated (albeit out of curiosity) and that was real life. Hentai with 6 tits, obese hentai, penetrated by bears, cars, trains, buildings, raped by slugs, roaches....hentai isn't necessarily based on real life fantasies either. One of my favourite h-mangas involves a little girl of around 8 getting stuffed with beetles, being f*cked to death, torn apart and rendered inside out and the twist at the end was the guy was mad at her for calling him a wierdo.
If you want to learn more about the more abstract and deviant aspects of hentai (and by virtue human sexuality) check out not4chan for loli (kids and babies) gurochan for guro (death, abominations, scat etc.) and of course the guro over at futaba which has a guro and animated guro section that focus on it as an artform which may or may not be in a sexual situation (gurochan is mostly sexual) also check out /d/ at 4chan for futanari (girls with penises and pussies) and other more mild deviances (like pocchari/fat girls)
I'm not linking them because google exists.