I'll give you some thoughts on the direct question "why are my family upset..." and assume that you are engaging positively in your choices when it comes to sex life.
I think there's a simple reason many in your family will puzzle. The same reason several answers in this forum are puzzled: sex is a major human drive, a factor of health for many, and a source of joy and connection with people they love. When told your choices, people think of their own lives, and feel that as they'd miss it and find it hard, you must also miss it and make the choice negatively - out of fear, pain or psychological unease. They worry about a life for you without this joy.
Then, specifically for your parents, they must be thinking about it with their experience of parenthood. Being a parent is a transformative experience. For me, I remember the joyful sleepless nights of early days and being exhausted, excited and never wanting to look back. I remember reliving my own childhood experiences with my children and it's a powerful experience, much more than sex. My own children will not have children, for other reasons, and it took me a long time to fully accept this.
So your family could be struggling to empathise with your choices and believe they are positive, be concerned that you are suffering / struggling and that would be the reason for your choice, and saddened that you are not just giving up sex - but also parenthood, two life experiences that were special for them.
Where to from here? To reconnect with your family regardless, you need to show them that it is a positive choice, and that in other ways, you enjoy life and its experiences, you are connected and happy, love them and to share your life with them.
Of course, if you also struggle to get out of your house, can't travel, have extremely few friends, no professional life... That's a bigger story. But sex is not the main problem then.