I don't think happiness merely depends on whether or not you have long term lasting romantic relationships. Does anyone here really believe that?
Whether you're mono or poly, a swinger, single or partnered, straight or queer, "happiness" depends on many factors. In my experience, mental and physical health or illness is a huge factor in whether one is "happy," enjoying life. If you've got cancer, or a bad back, or lupus, you're not going to be blissfully happy.
Another factor is the health and well being of your kids and any elders you are responsible for. If one of your kids is ill, very ill, it can consume your life. If you have an elder living with you, cranky or demented or unable to walk, how happy can you be? If you have a child or teen in trouble, or even a young adult, who is foundering, in danger of harming themselves or being harmed, how happy can you be?
A long term lasting relationship is great, wonderful. Asking for more than one is a lot to ask from the universe. I know a few people have 2 long term lasting relationships. My gf Pixi, for example, is happy with me (10 years this month) and has a bf she sees 2 or 3 nights a week, having built up to that relationship over the last 5 years.
I had a 30 year mono marriage. I had plenty, plenty! of happy moments, days, weeks, even years. We had 3 kids, we traveled, we made music and art, and enjoyed others' talents at shows and museums and libraries, we hiked, we swam, we fished, we gardened, we ate well, we owned 2 homes, we educated ourselves and our kids, we launched our kids. We have 3 grandchildren.
There is happiness in family life, but also many many challenges, and tears, and sleepless nights, and worries.
Hey, life is hard. Live and let live, I say! Don't be overly judgmental of anyone's lifestyle or life choices. Everyone has a story. Everyone has their weaknesses. We are all doing the best we can. Life is hard. That's why (almost) every religion has an afterlife idea where there are no problems or worries, just everlasting bliss. Because there is NO everlasting bliss in this life. Just gather all the pleasure you can, to balance the pain. Don't beat yourself up. If you fall, get up and face the next day. If you make a mistake, learn from it and move on. That's my philosophy.