Op... I am in an Open marriage.
My husband, of 11 years, who is monogamous is the one like you who opened things up. Before him I flitted from boyfriend to boyfriend. He approached me with the same original argument as yours. He wanted me to be happy.
BUT he had his own rewards in mind when he did so and it wasn't simply me being happy. He wanted something out of it too that I couldn't give him when it was just the two of us. He wanted BDSM from me and I am not wired that way. But if I am extremely happy I am apt to get more adventurous with hubby and his wants. He also is a bit of a cuckold.
Long story short I have met a man who I have fallen deeply in love with. Never went looking for the level of intimacy I have with M. Both emotional and physically. My relationships are separate but equal. I spend 50/50 with each man.
It is hard for my husband. He worries that I will leave him for M. Not happening. My husband is a once in a lifetime love. I love each man deeply. They are so different but yet the same.
Originally my boyfriend was to be kept separate from my family. Things have changed. My kids will be meeting the boyfriend soon. They will be joining me on some of my weekends with M at least during the day. His family is excited to be gaining three new kids. My parents and MIL live in Illinois and Iowa. My poor kids have no local family. They will finally get what other kids have, and my boyfriends mom is getting instant grand kids and is excited. My boyfriend never wants to get married and while he likes kids he doesn't want any of his own. Never felt the urge. But he wants forever but he is like me and needs his space now and again. I go home several nights a week.
For example this past weekend was M's weekend off. I spend those weekends with him. Usually from Sat afternoon through Monday when I go into work. I also saw him for a bit yesterday morning and I will going to spend the night tonight. This weekend he works and my husband is off. This weekend will be a family weekend. I wont see M until either an evening next week or the following weekend.
My husband is one hell of a man. I know he gets jealous. I have given him permission to find someone else. But it isn't always so easy. He does give up a lot for my happiness which I appreciate and cherish.