"Sex is best enjoyed when
"Sex is best enjoyed when you do know someone!!"
If this is an argument in favour of monogamy, I'm not convinced.
The case usually made for monogamous sex is that you can really get to know someone and get to know what they like and so on. The flipside to this, which no one ever talks about, is that you also get to know all the things your partner doesn't like.
Every time you go to bed with this person, somewhere in the back of your head, you're thinking about all the things you wish you could persuade him or her to enjoy. Oral sex, anal sex, rough sex, roleplaying, threesomes...
You tried it once and she wasn't into it. Or he's "just not comfortable" with the idea.
So your relationship continues on and you have great sex whenever you want, or at least fairly often, which seems to you like a decent tradeoff for all the fantasies that you will never fulfill. Plus you've gotten to know her body quite well, and you've discovered that you can make her come instantly by licking the back of her knee.
Great! Have fun licking the back of her knee for the next two years.