The one thing I have trouble getting past is "He's in a relationship".
If he is, does she know about this? If she doesn't, and doesn't approve, then finds out later, it could be a problem for the two of you.
As for the part about being extremely nervous. That's very normal!
Is it hubby that has the demand that he fuck you first? If so, I think that's a reasonable request. (Although I tend to avoid "demanding" anything of or from Mrs two4you

)