Melinda is actually portrayed as a slut, but someone who doesn’t know it yet. Hence why she fucked Daniel, and why she got turned after being fucked by the black dude, enjoying the sex even after Daniel is tied up, until she comes back to her senses, not to mention Lee grabbing her ass.
The sex with Melinda became boring because we got too much too soon. It was no longer a treat. If you have pizza every day, it no longer becomes what it once was. The writer had no Real ideas, it was just sex and sex, which at the start was amazing, but after that he should have had meaningful scenes. He didn’t, and that’s why the game is boring as hell. This is just subjective of course.