I think this one probably isn't flagged well enough in the game, you're not the only person with this problem. But the easiest way to trigger that flag is for Alex to give you a blowjob in the hottub only to be interupted by Lin's encouragement
Any particular requirements or parameters? Lol Cause like with Lina, I swear I've tried it from every possible angle but am just missing that one piece
The three requirements are that you get the Alex porn date set up, You get the thing wrong with their relationship and you talk to Lin about it. Then you will be able to invite Lin to the porn date.