I think it is important to first develop nonsexual, loving relationships before branching out into relationships that involve sex.
Sex is also not a constant in relationships... levels of desire go up and down and one must be prepared to respect that life is not just one big F-fest. Love does not equal sex, and that is what the family should model. If people acted on every single sexual impulse they had, this would be quite a messy world indeed. Not having sex with your family members helps to give you control over your impulses. People who can't control their impulses tend to be trouble with a capital T.
The family unit serves to help the individual learn about relating to others in a non-sexual way. When the family unit involves incestuous relationships, you end up with all kinds of dysfunctional behavior, mostly related to control and power issues.
I have known girls who had incestuous relationships with their fathers and brothers. Growing up, some of these girls were my friends. There are a lot of them out there, trust me. What I noticed about them was how willing they were to use sex to manipulate guys in general. They grew up seeing sex as a means of power and control and they wreaked quite a bit of havoc in their personal relationships.
If you don't believe me, just get involved with someone who has been a victim of incest. Find out for yourself just how difficult it is to deal with someone who did not grow up with nonsexual family realtionships.
Anyway, that's just my opinion but it is based on the undergraduate work I did in psychology and sociology.