I wonder how many of the half a million+ Mac botnet victims were told "no" when they asked whether their Macs were vulnerable?
OP, you're not the first asker of this question to get a poor response from this forum, and unfortunately you won't be the last. Those who nonchalantly dismiss the idea out of hand aren't doing you any favours. A better answer is very much "it depends".
If you only boot your Mac up on a Sunday to download your photos and send a couple of emails then no, it's likely not necessary. On the other hand if your Mac has any sort of business-critical function, connects to any and all kinds of external media and/or public/unknown networks, and is used to open lots of unsolicited mail, then you're very much in the "maybe" camp in my opinion.
Is it a minimum requirement for Macs to be running some kind of third party antivirus? No, I wouldn't say so. Your best defence is a well-rounded approach - good online practises, keeping your software up-to-date, configuring your OS properly, backups etc.
You need to decide for yourself whether it's worth the additional overhead (whatever that may be) in your situation. I would avoid taking Apple-centric forum advice as canon because it's often entrenched in old ideas and based on little or no contextual detail and massive assumptions. Case in point: this thread. Nobody has even asked you what version of OSX you're running.
Similarly a lot of "Mac malware" information you can find on the web is too polarized to be useful (it's generally either a complete whitewash, or a security company inflating the threat to sell software)
Do your research, but with a critical eye.