Main question: Is it unfair or unrealistic for me to say that he can't fuck her while I'm at work? That he can only do it while I'm out doing something social as well?
Yes, unfair and unrealistic. His relationships are his to run on HIS time, not yours.
If he is doing things in your SHARED time and not being present with you (ex: on a date with YOU he starts texting all crazy all night with another honey) then that's another thing.
I just don't want to be stuck at work while he's doing that. I want it to be that he goes and does her when I am out doing something (not necessarily with my lover, even just with friends) so that I don't have to think about it the whole time.
And what if you go out with friends and still
keep on thinking about them? The prob is not where your BODY is. It is where your MIND is.
Why does your MIND keep going there?