Choose 'Open relationship', then sleep with Doug, Jack and during cybersex with Eric tell him about that. Choose red talk options during that too. After Eric returns, deny him sex and he becomes cuck.
For Jessica GF path you need to not be in a relationship with any one and broken up with Eric. Plus kissed and had sex with Jessica on previous choices. She will propose to you when Eric returns.
To avoid being a Drug addict....never take the drugs even once.
No Nat Ending as of now. Hopfully it will come up in the DLC Eve is planning