Threesome?!

This may sound mad to others so please don't judge. But the thought of having a threesome with my husband and a woman is really turning me on. I mentioned it to my husband and he wasn't down for it as he said he would feel bad about sleeping with another woman, he soon changed his mind and is now all for it which really turns me on too. I'm just thinking in the long run now, obviously no one can turn round and tell me everything will be fine but, have any of you had any experience with this? What was your marriage/relationship like afterwards? What were your thoughts? 
I'm 21 and he is 27.