This is so coincidental. I'm reading The Quiet American and the main character is married and has a 20 year old Vietnamese mistress. Is it common for guys to get mistresses and cheat on their wives? Why don't these guys just do it with their wives? Eg, the reason why the guy in my book doesn't do it because he has to work in Vietnam
I think it is common, without looking at the statistics, from a multitude of factors. It is true, monogamy is a human made concept and our brains are not naturally wired to be monogamous, because we are naturally wired to procreate.
Other factor would be, a lack of sex life in a marriage, which can happen, in a long term relationship, whether you are married or not. Couples get complacent and the romantic spark will diminish if it isn't tended to. In the end, men need to have sex, which can also be true for women.
Another reason is that sex with the same partner can eventually become monotonous and mundane. Sex with someone else can seem exciting and refreshing, add that to the fact that the idea of it is also deemed immoral which adds a level of excitement to it. An example would be in The Simpsons, where Marge and Homer's sex life has gone into a rut, so they head to a hotel and they find that making out in sort-of public places creates that spark from the fact that it is considered wrong and they could get caught.
I think this is just a cop out.
There's more to a relationship than brain chemistry.
No one wants their partner to be complacent, and if you're in a relationship you have a responsibility to participate and keep it 'alive'
I agree with this. Even though, above, I believe what I said, I think blaming it on brain chemistry and biology is all just weak excuse.
There's a reason why we are the most intelligent species on the planet, and that's our ability to break away from our natural make up. People who use brain chemistry as a reason is merely using it as an excuse for not being strong enough to change, or not wanting to change.
That aside, if you don't want to be monogamous, that's fine. But when you enter a monogamous relationship, the onus is on you, not your brain chemistry.