If you have a commited relationship (meaning exclusive) you have every right to discuss this with her. It sounds as if this is an intimate setting and the person she's having sex with (making an assumption to answer your question) has an STD, you don't want to be on the receiving end of that. You have every right to protect yourself. You do need to be prepared for her to give you an answer you may not want to hear when you ask her if she's having sex with another without your knowledge and consent but in that case you are better off without her. Oh by the way, it doesn't necessarily have to be a man, so would that make a difference to you?
