I need help with my man...

Stephanie • Chef, art and cat lover 😊
My SO and I have been together a little over a year. We are extremely happy, always have the best sex, emotionally connected and are best friends.
But we got into a fight the other night. About our relationship and sex. He says that he wants more out of his sex life, as I'm the only girl he's ever slept with. And he hates himself for feeling that way, but he's got a penis, so what are you gonna do? 
Now I know (and he's emphasized) that he'll never cheat. And I trust him. He's asked about a threesome, and I've been considering it. 
I just wanted to know if any of you have experienced a problem like this before? Or if you can help out? 
The happy couple