I have two DDs - 5 and 18months. DD2's keyworker from nursery sometimes babysits for us. She is great and both girls love her. She has asked the last couple of times if she can bring her boyfriend (they live together and are engaged, she is about 23).
DH and I don't feel comfortable as we have never met him, are paying her to babysit (£7 per hour) and I dont think DD1 would be particurly comfortable (in fact she wouldn't because I asked her - we haveonly just started having a "babysitter" as opposed to friends or family). A couple of people have told me they think I am being a bit precious to tell her no she can't bring her partner.
She is fine with it by the way - I have told her that DD1 is a bit unsure of people she doesn't know and I wouldn't want her to be upset whilst we were out.