Why can't siblings get married?

does not always produce genetic abnormalities. Of course the risks are higher, but there are many cases where the offspring turn out fine. If we're concerned about genetic defects entering our gene pool, should we then regulate the reproduction of parents with down syndrome or spina bifida and other parameters that differ from the parental strain? Pro-gay activists attack anti-gay ones over the issue of reproduction, or lack thereof, how is the argument against intrafamiliar marriage over offspring that much different? Can siblings not be in love? Of course, many people are repulsed by the idea, but people are also repulsed by ity, and attacked for their closed-mindedness, can we not say the same about people who are against intrafamiliar relationships? Gay activists always talk about equal rights for all, the right to marry whomever one pleases. Why don't they extend that right to intrafamiliar relationships? If they do not extend that right, they have to right to say love has no boundaries. They are then to draw the line at gay marriage. What are your thoughts?