My wife and I have are married since 33 years, straight and we have a 32 year old daughter. We love each other very much, and for those around us we are the perfect role models for a relationship (only a few people know about our marriage model).
It is really deep love, combined with a lot of happiness and trust. Like happy siblings! We often hug and kiss, but it's not sexual, and has been for 26 years. My wife has her lovers (whom I don't know, we don't talk about them either), and I have my lovers. I always had the goal of having 5 lovers. Some I only saw once or twice a year, others once a week. They knew about the other women too, but no names. This worked well for many years, but at the moment I only have 3 lovers (I see one three times a year). I want to change that, my desire for intimacy and good sex is still strong.
Our marriage model allows for all that, it's bliss. I also think that we humans need sexual experiences and sensuality. How are you supposed to become a good lover, how are you supposed to recognize your own sexuality (and enjoy it!) if you've only had sex with one or a few partners? A woman once proudly told me that she had only had sex with her husband in her entire life. It seemed to me as if someone said they knew about literature - and had only read one book.

Marriage is also a curiosity: on the one hand it allows sex with your wife, on the other hand it forbids it with others. So many marriages are ruined by "cheating". But it is NATURE that triggers cheating. Your urges, your passions, secret desires, unsatisfied desires - they are also a valuable treasure. And sometimes they want to break out of the tight corset of marriage.