If a girl doesn't cum during vaginal sex, does the guy suck? Or do not all girls cum in sex every time?

Statistics and surveys have determined that about 70% of women are unable to have orgasm strickly from intercourse alone and require manual stimulation to achieve orgasm during sex. This has recently been backed up by a survey of the distance between a woman's clitoris and her vaginal opening, which revealed that the closer it is, the more stimulation she received and thus was more likely to acheive orgasm from intercourse alone.

That being said, if a guy is really bad in bed, then yes, it could be the guy's fault. Also, if a woman prefers and gets more enjoyment from a more endowed man, then a smaller guy may not do it for her either.

Side note: The other 30% that do have orgasms from intercourse are also typically multi-orgasmic and often times, very loud and vocal about their pleasure.