Monica and Richard are a divisive couple on Friends. Some fans feel that actually, they were a perfect couple, and that Monica's decision to turn him down in season 6 was a terrible one. Others feel that Monica and Chandler were the perfect Friends couple, and that Richard was a relationship best left in the past - or possibly not started at all.
The most contentious part of the relationship, of course, was the age gap - Richard being over twenty years older than Monica. He wasn't just older, though, he was a friend of her parents', which made a potentially reasonable age difference something much more concerning, for many fans. However, this wasn't the only red flag that popped up in the early stages of their relationship.
The Awkwardness With Her Parents
Meeting the parents is often a nerve-wracking part of a relationship - but in this case, was unnecessary, because Richard's parents had passed away, and Monica's parents obviously already knew him! Instead, Monica and Richard were left with an exceedingly awkward situation where they had to adjust to the idea of their friend dating their child - and it wasn't easy. This was never going to be a relationship that they were thrilled about.
They Were At Totally Different Stages Of Life
One of the biggest issues of the age difference was the one that eventually ended things for Monica and Richard - and it should really have been obvious to them from the start. Monica was still young, finding her feet, and wanted to get married and have kids. Richard, meanwhile, was divorced, and not only had kids but grandkids. These are not life stages that really line up for a successful relationship, given what each person wanted.
Monica Went To High School With His Daughter
Just to up the awkwardness even further, Monica went to high school with his daughter, Michelle. So not only would they have to deal with the fact that the Gellers weren't happy about the relationship, but Richard's family wouldn't be too thrilled either, presumably. It's hard to imagine someone being happy about finding out their father is dating the person who used to sit next to them in math class.
The Guys Saw Him As A Father Figure...
Having established that the families wouldn't really accept Monica and Richard fully, it's important to recognize that the friends wouldn't, either.
Ross was bothered from the start by the fact that Monica was dating their parents' friend, that they grew up around, and while Joey and Chandler liked him, it wasn't as an equal. Instead, they saw him as a 'cool Dad' and tried to emulate him.
...And The Women Just Saw Him As A Silver Fox
Phoebe and Rachel were definitely the members of the gang that liked Richard best, but they also didn't really see him as a peer, but as a silver fox type. Once again, it would be hard for Richard to really integrate into Monica's life. When Monica was later with Chandler, he was already part of that group (obviously), and when Phoebe met Mike, everyone treated him like an equal. But no one could just ignore Richard's age, and it definitely caused some issues.
The Gellers May Have Been Right About The Mid-Life Crisis
When the Gellers first learned that Richard had a 'twinkie in the city', there were plenty of jokes and insinuations made about Richard having a mid-life crisis. Jack Geller jokes about how he got the Porsche, and that's comparable to Richard getting a much younger girlfriend... and they may not be wrong. Richard is a man who is recently divorced, a new grandparent, and reveals that he finds his colleagues a bit dull and is basically where he wants to be with his career. Maybe he was having a bit of a mid-life crisis.
She Will Never Be Able To Connect With His Friends
Richard will never really be able to be seen as a peer by Monica's friends, and this goes both ways. Of course, she already knows some of his closest friends, because they are her parents, but it's hard to think of her hanging out with his other friends, either.
She is in a totally different stage of life, is still struggling with career and money, has no kids, and they refer to her as a 'twinkie'. Not really a good sign for building a combined life.
They Have Different Attitudes To Sex
In one episode, Monica and Richard talk about their 'numbers', and it comes to light that Richard has only ever slept with two women, and one of them is Monica. Monica, meanwhile, has slept with plenty of men (as Phoebe calls it 'a LOT'). Neither of these is a bad thing, of course, but the fact that they are so different points to some seriously incompatible attitudes to sex - and that isn't good for a relationship.
They Both Know It's A Problem From The Beginning
One of the biggest red flags in the relationship is that right after their first date, they both agree on all these issues, and how strange the dynamic and the age gap would be, and how inappropriate it is, and decide to not date. That should truly have been enough of a sign that they shouldn't date! Of course, they just can't get past the chemistry, and end up starting their relationship anyway.
He Knew Her As A Child
Even if it were possible to get past all the rest, this is the one that was the real kicker for fans. Richard has known Monica since she was a baby. He likely held her as a newborn, they talk about how she used to come and play at his pool as a small child, and he would have seen her go through all the school milestones alongside his own daughter. There's just no getting past the strangeness of that in a dating dynamic.