Who's in and who's out

Show understanding for your partner that he wants someone there, but clearly state that you don't feel comfortable with the person he chose. After all, it's you who has to lie there with your legs wide open (crude way of putting it, sorry...) to give birth to your baby, and in my opinion, you're the most vulnerable. Maybe look again together if there's someone else who feels comfortable for both of you to have there. Or just do it 'together'?