Well MichaelD I usually look at the sex in a black/white type of analysis: was it mind blowing/great sex or not? A good way to tell is if you could both stop what you're doing and just get it on anywhere, anyplace.
For me (even though I'm not married but have had 14 partners so far), sex is pretty dam important. If it's not mind blowing/great, I would have a hard time marrying her. The only girls I've considered to be worthy of marriage were great in the sack... Why settle? Also note that it doesn't have to be the "best" sex you've ever had but it should definitely keep you wanting more... and more...
Sure, a girl can have a great personality and be "compatible" with you and you both enjoy each others' company, but... if you aren't highly attracted to her (i.e. wanting to bang her at all times of the day) then you shouldn't get married IMO. Think about it, when you are having mind blowing/great sex you are on top of the world, add love into that equation and you're more likely to live happily ever after. Eventually, if the sex is average to bad, you are going to go looking for mind blowing/great sex on the side (I know I would but everyone's different).
Ultimately it's up to you to decide, but if you know that the sex (in conjunction with everything else the woman has to offer) is good enough to make you NOT go looking on the side then I say keep her. Anything less and you're selling yourself short.