Because people are naturally polygamous. Just because you love someone, that doesn't mean you won't be attracted to other people too.
Monogamous relationships CAN work. But go against human biology. Remove the jealousy from the relationship and it'll go a lot better.
If I go to upstate NY i'll show you what free love is all about. 
That is where we differ ... if I am attracted to someone I will remain monogamous and don't get attracted to someone else at all .. it's being satisfied that does that .. when I am not in a relationship then I can find others very attractive .. I don't do meaningless or wham bam thank you ma'am episodes .. I had enough of that when I was in my 20's
I don't necessarily see jealousy as a problem per se as much as I see disrespect as a problem ... if you truly care about someone why would you want to direspect them for some sexual attraction .?. and why be with someone if they cannot satisfy you .?.
I totally understand a lot of men see things differently .. I just don't see "myself" as needing to go out and hop about like a rabbit in heat trying out anything and everything that looks good
My brain is my biggest and most important sexual organ .. unless I am "attracted' mentally .. the packaging doesn't even register