i've dated multiple partners at once when my girlfriend opened up to the idea of seeing other people. she enjoys meeting new people and going out with them, but we also still love each other and being with each other, so we support each other when we've had other relationships or met other people. i have no problem with it so long as we're open about it and if one relationship isn't suffering because of another relationship.
people have different needs and desires. there is polyamory and then there is **** buddies, and people must learn the difference. there may come a stage in your life where you may want something different, there may come a time when you want a different sexual partner, and these are normal desires. people get so used to comfort and routine that they never shake up their life or change, and people see change in other people and possible relationships. i've met many poly people and their lifestyles are as unique as anyone's "ordinary" relationship.
i also believe you can't choose who you love, you just end up caring about someone deeply because of who that person is, and i don't understand this concept where you have to lock your spouse away in a metaphorical tower and hide her from the rest of the world because you're insecure or think she can only love one person at a time.
dating multiple people has its pitfalls, drama and time management being the biggest, but if you know what to expect from the beginning of the relationship and you genuinely care about someone, multiple relationships can work