As the first factor one relationship in particular comes to mind...
One specific relationship I had with a guy who was horny 24/7. He also kept flirting up people behind my back, even online, where I noticed it. I figured, that my monogamous attitude might not match well with his polygamous livestyle, so I decided to set him free and break up with him. Because I figured that either I would get unhappy by having to accept his livestyle, or he might get trapped by having to accept my livestyle. I figured the only way for both of us to get happy was to just remain good friends and not to tie each other down. I wanted to do it after a furmeet we both attended together, letting him enjoy this meet and telling him of my thoughts afterwards. Unfortunately a third party we met up with, who was our driver to the furmeet ahead of us, got involved directly before said meet, learned of my plans and basically blackmailed me to either break up right away to his benefit, or he would do it for me. So I did it before I even had any ideas on how to do it properly. It ended as suspected, with tears on both sides and with my ex starting to smoke again, despite having quit for health reasons a while before (which is something I still burden myself to this day). The third party swooped in, offered consolation and comfort, among (only towards my ex), among other things (free sex), so they got over it quickly. The next morning I woke up to both of them, standing hand-in-hand in front of me, awaiting my blessing for their new relationship (on trial). Confused and still half asleep I gave them my blessing and they were gone as silently as they had appeared. This action also kinda ruined my plans on wanting to maintain a friendship towards my ex, but more on that later.
When I realized what happened, I instantly knew they wouldn't be a good fit together and I admit, I was gloating over the fact that they were a pretty bad match for each other. Eventually they broke up again and I secretly was quite happy about that.
I never even put any blame on my ex, since I knew his nature. I put blame on the other party. My ex always tried hard to repair the friendship towards me, but he reminded me so much of the third party, that I never truely felt like letting him come closer to me again, because I didn't want to be reminded on how this whole relationship debacle went. And while I never blamed him for his nature, I would have forgiven him anyway after learning some details. Due to testicular torsion in his case, one testicle needed to get amputated, after which his libido fell to a more "natural" level and also his attitude to flirt up strangers totally halted and vanished entirely. I realized that his whole sexual behavior before was probably being influenced by his physical state alone (he had quite a lot of medical troubles at that time), and that if had known about this, I would have never pushed him away and would have learned to live with it.
Now I felt ready to let him become part of my life again, but unfortunately he died some time later (cancer is a bitch).
I realized at this point, that most of our relationships and sexual preferences are way too possessive and limiting to find true happiness. So my lesson was, that while I personally would be a monogamous partner, I shouldn't expect this out of my better half whoever this might be in the future. I would just be me and accept them for who and what they would be, otherwise I might get robbed of the opportunity to meet someone sweet and nice due to some old fashioned ideals that I figured got less important for me the older I got.
The second factor was that I entertained a friendship with benefits with a good friend for a while, before he moved countries and I was quite content with the openness of it all. He wasn't tied down by me and neither was I tied down by him. This made me more tolerant of the concept of open relationships and while I still might default to old fashioned monogamy in a relationship, I wouldn't expect the same out of any partner anymore.
I think that most of us (as in "us humans") are a bit too bogged down in our attitudes towards sexuality, either due to religious beliefs or due to societal peer pressure to become the "perfect" nuclear family. But the thing is, perfection is an unattainable ideal and might as well not exist at all, while many people struggle with their discoveries about themselves. Nothing is more painful to see from a distance than someone who is in heavy denial about who they are and what they want out of life. They could be so much more happy by simply be whoever they want or feel they are. Instead some guys think that they have to defend their masculinity by calling tears "liquid pride" or some people keep using that annoying "no homo" meme, even in situations where nobody would suggest anything even distantly homoerotical related to be going on. People who have a hard time to accept who they are and what they are (as in what sexual orientation they have) waste (part of) their lifes trying to justify to themselves as well as the outside world what they are truely are all about, out of fear of not being accepted or of being too different.
And I get it.
There are many intolerant people out there. Even here on the forum (if discussions in the dumpster fire subforum are any indication). People are too preoccupied with a "us vs. them" mentality trying to defend their position in the "culture war" and trying to figure out if some other person is friend or foe due to their opinion, that they all miss the things in life that truely matter. And sometimes that is not to be too judgemental of others and to live and let live, which also includes sexual orientations.
That's for example, why I don't care much about lables like "bisexual" or "pansexual". The former is the attraction towards both sexes, the latter is the lack of specific attraction based on any sex or gender. Sometimes I AM attracted towards a specific sex. Other times I don't. What does it truely matter? It matters little to me, as I could still be sexually active with one sex, while being currently heavily attracted to the other sex. My sexual performance towards the hypothetical partner would not change due to that, and why would it? Therefore, why would it matter at all?