It absolutely wouldn't be for me, even the thought makes me want to be physically sick.
Saying that to be honest I think the lies of an affair would hurt me more than the physical act - I think - actually I don't know lol.
I couldn't let something think they were having one over me and lying and cheating whilst I had no idea what was going on.
I also think you shouldn't be so secure in thinking he won't leave you because it's been 2 years and he hasn't YET!!!
My best friend was having an affair with a married man for 6 years!! In that time the wife found out and whilst she didn't ignore it she knew it was happening and was desperate for it to stop but financially didn't want to leave her husband. The man was obviously telling my friend the marriage was over they weren't having sex etc etc - the wife then became pregnant (which was of course an absolute one off).
My friend also became pregnant during that time and was pretty much forced to have a termination. It crushed her.
After 6 years of him telling my friend he would leave etc, my friend - who was completely in love with this man, couldn't take sharing him with his wife and moved to the other side of the world to get away from him (she had tried to end the affair many times but always went back.
It was at that moment the man had to make a choice - his wife and children or my friend. He chose my friend, and after a 6 year affair he left his wife.
My point is - Perhaps right now the other woman is also content with the arrangement. However what happens when she's not? When she decides she's worth more than whatever it is she's getting from your husband. I wouldn't be so sure he will choose you over her.
If you are genuinely happy with your husband sleeping with others then tell him. Reassure him he doesn't need to leave her or any of his other girlfriends, and he can continue to get his happiness from somewhere else as long as he cuddles you on the sofa! Have an open relationship. If that works then it's fine.
Right now your husband doesn't care about what works for you, he's going to sleep with someone else regardless because he wants to. At least if it's open then there's no lie. He can remain happy sleeping with you and her and maybe others you are yet to find out about. You can carry on as you are or also find happiness elsewhere.