Unfortunately you can't choose your teenager's friends, no matter how much you may want to. So try to look on the bright side: Your daughter may actually benefit from her friendships with these boys since she'll learn how to relate to the opposite sex platonically before becoming romantically involved with one particular guy. What's more, the better your daughter understands the way boys think, the better she'll be at competing with them academically and, later on, professionally. So allow her to have her guy friends visit your house and get to know them. Chances are you have nothing to worry about.
If the situation really bothers you, encourage your daughter to get involved in activities that offer social opportunities for both genders and see what happens. Or you could casually ask her if she'd like to invite one of her girlfriends for a sleepover or on a family weekend trip. Just avoid criticizing her choice of friends or demanding that she see less of her male pals, which will likely backfire and push her to defy you.
Of course, if you're concerned about her being sexually active with these boys, then a serious talk is in order. Again, don't judge her choice of friends, but share your views on the subject and give her plenty of time to share her thoughts. If you don't like what you hear, try not to get upset (she may just be trying to shock you) but commend her honesty and say that you disagree, but that you love her and that she can always talk to you. Consult a health professional about obtaining reproductive health services for your daughter if needed.
Remember, teens do lots of experimenting. Chances are this "boy-crazy" phase won't last long and your daughter will soon discover the fun and camaraderie of being with other girls as well.