A few points:
1. $13 million in 2013
So what makes it worth $20 million today. Indeed was it even worth $13 million in 2013. One presumes the value was based on the presumption the domain would gain value over the years.
Many factors could suggest the value has decreased. Such as phone apps now very popular. In 2013 most people were using websites. Today I suspect app use is huge. We have a LOT more domain types, such as dot this and dot that. In other words supply and demand. The more domain extensions means less value.
2. 1.5 million unique visitors per day. This may or may not be because of the domain name alone. The site (that does not look that good)
https://www.sex.com/ looks to be some sort of free tube site. So it could just be popular because of the free content.
The question that should be asked is how much the site makes in profit and how much does it cost to run.