What does he think he is missing by not having sex with others?
First woman - You put your knob in, move it up and down, cum, job done.
Next woman - You put your knob in, move it up and down, cum, job done.
Next woman - I think you get the hint.
Between your husband and yourself you can be as experimental as between him and any other woman. In the end there is no difference and you he is not actually missing anything.
How many different woman is he allowed sex with? Each one increases the risks of you catching an STI. Will he and the women agree to both have complete STI checks before they start?
You may think you want this but how will you feel when you next get into bed with him.
If he tries something new he has learned from another woman. How much younger, fitter, tighter, more flexible than you is she? Did he kiss her the same way? Touch her the same way? When did he last shag her? Can you still smell her on him? Is his knob crusty from her?
Do you have children? Will they be introduced to 'Aunty Shag Buddy' as she leaves in the morning? Or as she comes down to breakfast? Will it be in you bed? Who will wash the sheets after?
What will you do when he prefers one of them to you and wants to leave? If he hasn't already got one/two/three lined up where is he going to find them?
How did he approach this with you? I think as with a few others there is someone waiting in the wings and this is his way of having an extra marital affair with little wifey offered the sames chances because he know you wont.