It's actually really interesting how Mike and Rachel really just represent something for the other. Rachel desprately wants to be absolutely brilliant and have overcome struggles and diversity... when really she's incredibly spoiled and privileged. She's smart, but she's not Mike level brilliant. Her biggest adversary in life was that daddy may have wanted a boy and isn't great at connecting with her but still treated her like a princess and gave her everything she ever wanted. Yeah, she wanted to live on what she earned... which is not really hard when mommy and daddy paid for college and will bail her out if the need really arises. She throws all sorts of fits when Mike can't give her her dream wedding. And you can already see her getting unbelievably jealous of him in the show- with Harvey of all people, who she doesn't even really like. That's not going to go away when she becomes an attorney. She's always going to harbor some amount of jealousy for how well he does, especially if Harvey is involved in any way. She's constantly putting him down and throwing the secret back in his face, not because she's afraid, but because she is jealous.
And Mike is... at the end of the day, just incredibly lonely and desprate to have some stability and someone to start a family with. Rachel is pretty and smart. She admires him. They have a lot of physical chemistry, but apart from law, really have nothing in common. But he feels like someone that Mike should want if he were a real attorney, the perfect princess. And you can't tell me that he doesn't walk on egg shells around her sometimes. He doesn't trust her completely either- he's very afraid that she's going to out him if they end on bad terms, and to be honest, I think she's vindictive enough to do it, especially if it ends because Mike picked Harvey over her for the upteenth thousandth time. I think it's a big reason why he went back to her after she kissed Logan (that and Donna making Harvey do it). Getting engaged to Rachel always felt like Mike saying "Well I made this bed, better just lie in it because I won’t find anyone else who will put up with my secret."
The first time you watch season 5b, you're legitimately wondering if Rachel is going to cave the entire time. She's far and away the weakest link, the easiest to get to, she's already proven she can't stay loyal unless Mike is constantly by her side, and Mike knows it to the point where he panics every time Gibbs glances in her general direction. And she wants Harvey GONE, bad, no questions asked. I have no doubt that if she could have thought of a way to betray Harvey for Mike without Mike realizing it, she would have done it.
And then presumably because she's too good to show up to prison, Rachel does not visit Mike once. Meanwhile Harvey is there. Every. damn. Day.
And then he marries her primarily because he feels like he owes her for putting up with the whole thing.
Just... so many toxic layers to their relationship, even without the cheating and Mike constantly picking Harvey over her. They should have broken up in season 5b and been done with it, let her leave the show by running off to Stanford.