It means that he wants to have a threesome with you. You both might have been together for a long time and maybe your husband had gotten bored of the same old things and he wants to spice up things in the bed maybe that's why he suggested that to you or asked you. He must have seen it on porn sites and he liked it and he wanted to fulfil his fantasy. And maybe he gets aroused seeing you having sex with someone else it's called cuckolding.