In order to fertilize your Drider eggs, I think that you have to have vaginal sex with the Corrupt Drider from the swamp. Specifically, you have to take her cock in your pussy, then they will be fertilized. I haven't found any other sexual partner that seems to work. When it comes to Urta I don't know if there is any difference whether or not the eggs are fertilized. When I filled her with unfertilized eggs, the next scene I had with her was her complaining about what she was going to do with a bunch of baby spiders running around. Let me know if you happen find any different scenes though, I would be interested in seeing them.