And this is where so many people make a mistake.
They think that because their SO says they love them, wants to move in together build a home, have kids etc that they can expect fidelity - wrong ! This is just an assumption.
If you want monogamy/ fidelity/commitment/exclusivity then you need to be sure that you are both talking the same language. And that means asking the correct questions.
I dated a guy and we'd got to the stage where he wanted me to move in with him. I asked him straight if this was what he wanted and if we were 'exclusive', and if he planned for it to stay that way.
His exact response was "I love you Bluebells, don't worry, I'll never take up with another women as long as I'm with you."
So I took that as a "Yes" and put my house up for sale.
Some time later I went round to his house unexpectedly and caught him with another woman. I went ballistic and called him a cheat and a liar "You said you wouldn't take up with another women as long as you were with me"!
His response "I didn't lie, I didn't take up with another woman because I was already seeing X when I met you".
(She knew nothing of me BTW)
He was lucky I didn't punch him on the nose.
I learned after this not to assume anything and to ask very specific questions.