This Was Supposed to Be a Pure-Love Adventure, So How Did It Turn into a Battle Royale?

The story was supposed to go like this: I transmigrate to a fantasy world, save a shy succubus who's terrified of people, and feed her some barbecue. She says, "Wow! You look so handsome grilling those kidneys! I want to stay by your side forever. Please don't abandon me! Boo-hoo!" Then she falls completely in love with me and marries me. She's a genius mage, I'm a beefy powerhouse, and we hit it off and go adventuring. I'm melee, she's ranged. I tank, she heals me. I deal damage, she supports me. We conquer dungeons, slay dragons and claim their treasure, rob the rich to help the poor, defeat the Demon King, become great heroes and make a fortune, then retire in triumph, buy a peaceful estate, have lots of kids, and live out our days in bliss. Perfect!

...But halfway through our journey, you suddenly tell me that this succubus is the Demon King's betrothed, a living Holy Grail, and descended from the Old Gods; that whoever wins her will gain the right to become a god; and that all the big shots are fighting to the death over her... And then she marries me, gives me her virginity, and signs a soul contract with me!?

...I came here for pure love, not to play battle royale! What are you people doing? You damn bastards, anyone who tries to steal my wife dies! Pure-love warriors are invincible!

...Anyway, this is a vulgar story about a beefy guy who tears through the whole world to protect his pure love.