Can't masturbate without this stimulation
That's like you say with the sex though OP. There's something not quite right with him. Most people, even men, can get off without images. We might choose to use them sometimes but we can also get off without them. I had a lover who could virtually only masturbate, particularly if he was by himself, with porn and that's because he really struggled with his sexual response/orgasm for health reasons.
Lives with us temporarily because she couldn't quarantine on her own
This is only temporary though. You (and if necessary, your mum) could think of another plan sooner or later. A lot of private landlords who handle their own affairs will still be renting now.
the kids wouldn't cope well
You don't know how they'll cope till it happens, I'm sure you'd support them well. It happens to about 1 in 2 families.
Hubby wouldn't cope well. He is so very loving in many ways and also an old romantic.. Always talking about our history, so full of love. It never seems to extend beyond words though. I don't "feel" it. Maybe its me
No, I think this 'sweet talk' is a way of manipulating you and getting away with the other stuff.
My eldest son cannot stand his dad
Then he will be glad not to have to live with him, as I felt about my dad when my mum left him. I was constantly telling her she should leave him, till one day she turned round and said she'd rented a house. 
Everything you say about how the kids feel about him suggests they would in fact take no longer living with him very well indeed.
person i am having "sex" with
If you can see that 'sex' has to be in quotes, then it's already over in your mind. That's how I feel about the 'sex' I had with my ex- and he's an ex.
He says this position is just easy, requires no effort
Is that supposed to sell it to you above all other considerations?
What happens when you say you'd like to come sometimes and this isn't doing it for you? Or that you don't want this 'sex' because it isn't pleasurable for you? If you ever want to talk to him (at least in future after Corona) maybe go out for a walk or something so he can't kick off to avoid a conversation. Not that I'm under any illusions about his willingness to change.
You can do it @mamato3lads . Trust me, life's so much better without 'sex.' xxxxx