It has been reported that while 75% of men reach orgasm during sex, 29% of women report the same.  Additionally, most women are unable to climax through vaginal intercourse and ultimately need clitoral stimulation to reach orgasm.

For me, sometimes during intercourse I don't have an orgasm, but if the man stimulates me enough during foreplay it can and will happen more often.  I love gentlemen who take the time to please the woman he is with before he climaxes.