Yeah you know, gradual. Evolution can result in amazing things due to the tiniest of mutations and the most marginal of pressures, over time.
So what, eating food isn't just about getting nutrients but we still instinctively avoid eating things that repulse us. Sex is involved with reproduction (um, clearly), and that's enough. Certainly enough to confer an advantage on the offspring of those men and women who do not have cock-in-vag sex with close relatives, I would suggest.
Actually, food repulsion patterns are very particular to the individual and are based on larger cultural factors and cultural traditions. Most Americans would gag trying to eat fermented tofu in China, for example, but there it's a delicacy.
As for the rest,
What you're saying makes no sense. It's a very commonsensical sort of misunderstanding of what sexuality is with regard to evolution that uses this sort of metonymy where intercourse stands for sexuality, and vaginal intercourse = sex.
There is no reason why people should be "repulsed" by people whom they are not attracted to. The way human attraction works is not according to some "attracted/repulsed" binary. People are not solely attracted to people who would make good reproduction partners. As is evidenced by many diseases (sickle-cell anemia, tay-sachs, etc) that are common in certain ethnic groups and are the result of a man and a woman (who are attracted to each other) reproducing.
Evolution, unfortunately, has not selected out attraction between people who may not make a good genetic match. Attraction is based on many factors in addition to things like pheromones.
There is no reason to believe that people who are sometimes attracted to relatives close enough to be genetically disadvantageous as reproduction partners would not be "preferred" by evolution. In fact, looking at population patterns, we know that many thriving societies were full of the offspring born of first cousins and other family members. This is especially true of pre-modern societies when people were much less mobile than they are now.