Monica was right about Mark all along!

Remember this episode takes place before Chandler's Christmas with Wendy and it has nothing to do with that, Monica and Chandler weren't even a couple at that time or on the horizon for a relationship so don't bring up, "But Monica didn't trust Chandler when he was in Tulsa with Wendy:" It's completely different, has nothing to do with that and for once, let's stay on task people!

This is all about Monica's reaction to her brother's behavior and immediate dislike of Mark and lack of trust in Rachel. Toss should have realized that Rachel is familiar with the word no and he should have realized that not every woman is out to cheat on him the minute she talks to another man and it was just a conversation at that. She told Toss that Rachel had a golden opportunity to get her foot in the door and since it had nothing to do with Toss, he should trust Rachel; just trust her to do the right thing; but Toss was acting like Rachel was just like Carol and that Mark was a male version of Susan but Monica told him; Even If Mark Wants To Sleep With Rachel, Does That Mean Mark Gets To Sleep With RACHEL!?"

What ended up happening in the long run?
Mark was in a relationship with someone else. Rachel knew Mark's girlfriend all three of them worked for the same company and Toss spied on them lurking in the hallway hoping to catch Mark and Rachel red-handed in a compromised position; instead, he caught Mark in a compromising position with his own girlfriend, with someone else! Yet he still made an ass of himself and assaulted them, refused to apologize and threw the love bug at Mark. Mark then quit later on in the week and told Toss that Rachel would have less spare time to spend with him because she has to pick up where Mark left off. [I think personally that both Rachel and Mark worked out that arrangement together and called in Toss because they wanted to cover all bounds and make no misunderstanding that Mark isn't going to work there anymore because they were both too frustrated and beat down by Toss' constant interruptions putting both of their jobs in jeopardy] figuring that if Toss heard it from both Rachel and Mark and saw Mark leave with his own two eyes he'd stop giving himself a stroke and harassing the two of them and assaulting people. Meaning the fact that Rachel had to get called into work on their universality shouldn't have come to a surprise to Toss. Mark quits his job. Toss and Rachel break up, and Mark and his girlfriend probably broke up too and it's never said what happened to her, but after three weeks Rachel goes on a date with Mark. Mark admits to Rachel that he had a crush on her and it was love at first sight type of crush, and he wants to have sex with her on the couch.
What was Rachel's response to that? What was her reaction?
She immediately ended their date and kicked him out of the apartment! You want couch sex? Date's over! Meanwhile, she already felt guilty for dating Mark when she saw Toss sulking in the hallway and Toss almost made it to their location to spoil the date but Chandler stopped him in time and Rachel admitted she was unfair using Mark to get back at Toss but Mark spoiled it all himself by declaring, "Ah, Fair Schmaire! I say we get back at Ross right here on this couch!"

What happened was, Monica had already told Toss that there was a chance that Mark wanted to have sex with Rachel but he probably won't get to and Mark never had sex with Rachel!