You fire someone because they don’t do their job. They sue for discrimination. How do you prove it?
The lawyers are drooling.
By having a paper trail showing that they weren't doing their job and that they had been warned about it.
Thorough performance/attendance/workmanship documentation is how you prove it. Prohibiting workplace discussions of a sexual/sexuality nature and avoiding engaging in them yourself greatly reduces the risk of anybody being able to prove the decision was based on anything other than professional reasons.