i may be a bad example here because im a netizen that is well traveled in porn sites ( I feel like you have to be to even stumble upon pregchan), but your fantasies are not degenerate. in my opinion they are certainly strange and unusual, but not degenerate. the pregnancy/breeding harem is also something i fantasize about although to a lesser extent. adult birth is definitely something im not into, i don't think it's degeneracy, but just not my thing. i've looked at unbirth and soft vore content before and while i like seeing the giant belly stuff, i just dont think it's 100% for me.
this one is kind of specific but I think my most fucked up fantasies involve a woman i know personally (not always the same person) getting their body transformed in such a way that they feel like they can't go outside anymore, and their partner breaks up with them. for example immobilizing giant breasts, extreme weight gain, growing giant penis, etc. then i swoop in and enter a relationship with them, telling them that they're beautiful to me, but i just enable them to forever stay in, and be exclusively mine. i would impregnate them and have a ton of sex with them. their body would grow bigger, eventually requiring a new home and she would be unable to care for herself
I don't have to use this fantasy every time i masturbate, but its certainly a strong one for me. I particularly enjoy breast expansion, and pregnancy, so those are strong aspects in this one