For a lot of people, sex is an essential part of their lives.
Obviously it's not essential in the same way eating, drinking, sleeping, breathing are.
But most people would consider regular human contact in some form essential.
For most people, if they don't have another human to talk to for a couple of days, they'll start to go a bit stir crazy. If you never get a hug off anyone, it takes an emotional toll on your mental health.
For a lot of people, sex is at that level. I'm not physically going to die if I don't have it, but it's not good for my mental health.
And no, masturbation doesn't cut it in the same way that talking to yourself could never replace a genuine conversation with another human, or that stroking a cat doesn't replace a hug from a person. The physical release of orgasm is nice, but it's not the point. It's about sharing something with your partner, with another human being.
The problem is that sex is the one thing that most people can't do with anyone else. You can have a conversation with anyone, you can hug your kids, your Mum, your best friend, but most people don't want their partner to have sex with someone else.
Which leads to a lot of people in relationships with mismatched sex drives who either aren't getting enough sex, or are being pressured into having sex.
My DP is a bit like you OP, she enjoys sex, but it's very low down her priority list. So we muddle along, I get less sex than I'd like, but I don't pressure her, because there's nothing less attractive than someone begging for a shag.
For her part, when she is in the mood she makes sure it happens, and I try my best to make sure the mental load is shared, as she's far more likely to be in the mood if her mental to-do list isn't a mile long.
But she knows that I'm not willing to be in a sexless relationship, or in a relationship where the other person is only having sex to keep me happy, and every few years things start to slip so we have to have a conversation where I express that I'm unhappy with our sex life, we talk about what the blockers are and move forward from there.
You don't have to have sex that you don't want to OP, but equally your husband doesn't have to stay in a relationship where his needs aren't being met, so your best bet is to try and work out what'll make you want more sex, and work on it together.