I have recently taken to keeping a single male betta in with my smaller goldies in the 46 gallon, and also my "floaty" Ryukin in my 20 gallon. I haven't encountered any problems thus far, other than the 2 males trying to fight one another through the aquarium glass [lol, the tanks are side-by-side].

Is there any reason that I shouldn't keep bettas with my goldies?

The temps in the tanks are room temp.

Hola Cheekun... And I would say.. it didn't work out for me, but that was because my goldfish killed my betta. @_@ Not the betta harassing them.

It lasted for a few days, maybe a week or two.. hard to recall.

Some people can keep betta fish with goldies successfully. The BIG probblem I see is that in a 46 gallon your little guy will have to swim really far to the top of the tank. He might do better in the 20 gallon.

Been there done this,and in the end,it really doesn't work so well.First thing is that Betta's prefer warmer temps.
2nd) Normal filtration rate for Goldfish is a lot more powerful than Betta's can handle to constantly swim in those currents.
3rd) Goldfish are pigs,plain and simple.Betta's often will go without correct amounts of food because the goldfish will gobble theirs up.
4th)Sometimes there's a stress factor going on because of size difference in Betta's and goldfish.Eventually,one will harass the other.
This seems to work for a while,but then your Betta usually pays the price.I kept one for a while with smaller goldfish that ate all his food.And then one day I happened to watch him in the tank,and that's when I realized how much the filtration rate was bouncing him all over the place,and how much effort it took for him to try and just normally swim,and saw that he was totally exhausted by it all,especially when it was time to come up for that breath of air.
My advice to you is that if you're able to,give the Betta his own space,as he deserves to lead a peaceful,easy going kind of life.

I wouldn't mainly because of the temp preferences.
A friend of mine kept three little goldies with a betta. The BETTA killed the goldfish! But I'm sure that isn't common. I would get a 5 gallon for the betta. It'd probably be happier there.

In my experience, my betta ate my moor.
Well, actually, I don't know if he ate him, but I'm pretty sure he disemboweled him well enough.

I agree this is a bad idea. The temp is a huge thing with Bettas.
What is "room temp"? While I am no betta expert I found room temp just not to be enough for my betta. I believe they thrive at high 70's, I mean like 77-79F.
Not to mention the picking at gf tails and fins.

So far, I haven't had any troubles. The Bettas find the most peaceful waters to swim in, and I've seen the Bettas eating well. The goldie in the 20 gallon was lonesome, but all my other goldies are a little too boisterous and large to go in with her. I'm going to watch for trouble constantly, but things are ok so far.

Room temp for my room is between 74 and 78, with it sometimes climbing into the 80's when I'm not there to make sure.

We can only warn you from what we've done ourselves,from past experiences.Hopefully ,that won't come true for you.I have found though through experience,that sometimes situations like that are only a matter of time---they are like waiting for a bomb to explode. It only takes one time for disaster to strike......If there's ever any slight chance that a fish may be harmed at some point,well,to me it's just not woth the risk---but that of course is JMO.... All the best luck to your little Betta.