erm...did you ask either of them for his phone number? I know I wouldn't think to offer my husbands phone number if nobody specifically brought it up that they wanted it. She might have no problem at all, and if she does, better to know ASAP.
But yes, I am sure I'd say "I know I said I wouldn't date other people, but I need to have a sexual partner, so I have to know if I will be able to have sex with "whateverhisnameis". If not that is fine, I still would like to be involved with you but I'll be dating other people"
I don't think you should be focused quite as much on worrying YOU are going to make her uncomfortable, focus on making yourself comfortable! (Anyway I tend to think that a good 50% of new relationships is having awkward conversations!)
Happiness will never come to those who fail to appreciate what they already have.