It's not a binary question
Whether humans are wired towards emotional, romantic and/or sexual exclusivity, is a complex but for me an ultimately settled question. To summarise a long debate, humans are a product of both nature and nurture, born with a blueprint of unique genomes which can be switched on and off by many different interactions and environmental factors. Whilst we have a tendency to pair-bond, some people are better wired towards exclusivity, whilst many others aren't... and it's not binary.
To strive towards one end of the spectrum and define it as the 'only right way' seems utterly unreasonable and a waste of time. Life is short. Better to find people and relationship styles which work well for you all.
But even if you are wired towards the non-exclusive end of the spectrum (in any or all of the three areas) doesn't mean that a non-monogamous relationship is suitable for you. How you cope with jealousy/insecurity, how you view commitment, community, honour, lying, as well as your cultural background and philosophical beliefs to list just a few, will all play a part on how you choose to design your relationship. And all those factors may change over the course of a lifetime.
Even ostensibly monogamous relationships tend to allow for deep emotionally intricate friendships. Sometimes this is enough to satisfy a non-exclusive emotional inclination. Perhaps some people with little effort, can resist their non-exclusive inclinations and their reasons for doing so are their own.
You can also choose to express non-exclusive inclinations in other ways; serial monogamy, romantic (but non-sexual) friendship, cheating, don't ask-don't tell, swinging, friends with benefits, hotwifing, cuckolding, concubinage, polyamory (hierarchical and non-hierarchical), polyfidelity, solo/singleish polyamory, relationship anarchy, and more. Some are more 'ethical' than others, but ethics are both culturally and personally defined. What is ethical in some cultures is morally unjustifiable in another.
What makes one a more attractive option than another for you? You are the only person who can know, and such a process of self-discovery tends to occur through trial and error. Yes, it may hurt.
A Case for "Don't Ask-Don't Tell"
Let's say you have a cultural or familial background, where giving wrong information is not considered lying. Nor is it necessarily considered a bad thing. Lying is perfectly acceptable - and even respected - when honesty will conflict with the higher cultural value of communal honour, family stability and/or harmony. In this particular relationship, in this particular culture, it is tacitly accepted that sexual expression is a necessity but compatibility not always a given. Love connection is not as important as formal marriage. Harmony is better kept through lying (or plausible deniability) than through telling the truth.
Polyamory - which is difficult to hide given that it usually espouses deep emotional and sexual connection as well as open honesty - may not be an appropriate choice. The risk of outside discovery might hold the penalty of disgrace for the entire family. Where communal honour, family stability and saving face is held as the highest value over honesty, perhaps the best survival tactic to express a non-exclusive inclination is a (pre-agreed?) discreet, don't ask-don't tell configuration, bearing in mind that outside parties are not only 'needs fufillment machines' and that their participation in such an arrangement should be conscious and consensual.
A Case for "Same-room Swinging"
Let's say you were brought up in an extremely high achieving, large family, where being the best mattered to your survival. It's created a deep need for appreciation, coupled with an underlying insecurity. You therefore adhere to the notion of natural 'hierarchy' and division of labour within your relationships. Let's also say, that you believe in a set of values which include emotional monogamy. You are yourself, emotionally mono-amorous in nature... and find that if you fall in love with one person, it supersedes your feelings for someone else.
But let's also say that you are highly sexual and enjoy different flavours of sexual expression. Lying - even by omission - creates huge suspicion and stresses the insecurities, so 'don't ask, don't tell' wouldn't work for you on that basis alone and neither would cheating. Hierarchical polyamory conflicts with your best-known survival strategy of being number one. Perhaps the happiest option for you to express your non-exclusive inclination is 'same room' swinging... whilst working hard to improve your self-esteem.
A Case for "Relationship Anarchy"
Let's say that like the last case, you have a high intolerance for lying. Let's say that you have recognised your abiilty to love several different people at once in several different ways (all hail to the Greeks for naming six different ways; eros, agape, philia, ludus, pragma, philautia) and your parents' divorce has left deep scars fostering a high priority on community as a fundamental need.
You therefore like the idea of non-hierarchical configuration (as per polyamory) but prefer one which allows a community or network of friends and family where the relationships contain mutable but unranked components. You are against rules based relationships and your relationships are allowed - even expected - to transition between friendship, romance and/or sexual attraction and perhaps back again, over the course of a lifetime. You aspire to freedom and trust over duty and loyalty. Your best option to express your non-exclusive inclination may be Relationship Anarchy.[NB. Since writing this post, I've explored why relationship anarchy may be better termed "Relationship Fluidity" if your philosophy is not grounded in the principles of political anarchy]
I like to call the different relationship expressions 'strategies' that you use to get your relationship needs and preferences met. The very act of discovering how emotionally, romantically and/or sexually exclusive, or non-exclusive each of you is wired (and why), can be a tremendous source of connection - once more, bearing in mind these inclinations are not binary, and can be fluid over time, with different people.
How you choose to design your relationship together is a wonderful work. Some strategies conflict with one another, whilst others do not. A person who prefers exclusivity themselves may be highly sensitive to lying, but prioritise high autonomy, space and care little for 'saving face'... which makes them better able to cope with a mono-poly combo relationship. Or they may prioritise safety, consistency and inclusion which means they cannot perceive their partner being anything other than monogamous with them, whilst acknowledging that the definition of monogamy itself, also changes from person to person.