As a animal why would I need to goto the effort of pleasing the opposite sex if I get all the gratification for free with porn and without the stresses and hassle, of an expensive marriage and nagging?
You don't get "all the gratification". Since you are focused on the purely physical aspect, I'll talk about that first - it's not an equivalent orgasm, scientifically speaking. Studies have shown that the quality of orgasm is around 3-4 times better with a partner, as measured through hormone levels post-orgasm. I forget exactly which hormones, though I believe they measured at least dopamine and prolactin. Possibly testosterone or something else as well, though I'm not going to search about orgasm hormones at work to find the study, haha. If memory serves, your dopamine (the thing that makes you horny) plummets and your prolactin skyrockets, giving you that post-coital sense of bliss. And the amount dopamine/prolactin fall/rise, are 3-4 times the amount than when it's just you and your favorite hand.
And that's not to mention the mental or emotional aspects at all. Nor anything about a relationship (if the sex was with a longer term partner).
Porn is about fantasy and serving a short term need/desire. And it's a form of entertainment, fantasy entertainment. No one expects real sex to be like porn, just as no one expects action spy movies like Bourne to be like how intelligence agencies operate. That doesn't mean that regular/real life isn't worth living or doesn't offer something of value that isn't covered by the other thing.
It's difficult for regular guys to experience mindblowing sex with a woman that is giving them everything like they see in movies - they should be happy if they get their 5-10 minutes and then move on. So what they see in porn they never get to experience in real life and it might be significantly more enjoyable to them than the real, yet average thing that is available to them. I guess that also explains sudden interest in realistic dolls (can't shake the pathetic feeling from that). Some people here do experience the jackpot; if they are smart they don't talk about it, the rest is just theorizing what if.