If you love this girl, then no way would I advise you to allow this. If she's just some chick to you then I guess you could go along with it, but I would not. Like other have said, you never know, she might decide she likes being with chicks better than being with you... not the mention all of the health issues that could arise. How old are you guys? Did the two of you have alot of sexual experiences before you got together? Cause that can make a big difference in relationships...
Good luck man, I hope it works out for ya...