I think when you have to start talking about mandatory psych evals before relationships can begin you're talking about a situation where the alternative of the couple having to find others to be with is more viable than doing what it takes for them to be together in a "healthy" way. Premarital counseling is certainly something that I think most professionals would recommend for any couple, but if it's essential, then there's something of merit in considering the possible flaws that may arise as reason for keeping the practice outlawed.
Any relationship that needs a psych eval to begin is probably doomed. It's sort of like those relationships where the people get together because one person cheated on their significant other with them. They then get together not realizing a relationship that required adultery to begin is all kinds of tainted.
There is also the very real fact that an older sibling or a parent can indeed have a bad influence on their child/younger sibling.
__________________ Chicken Boo, what's the matter with you? You don't act like the other chickens do. You wear a disguise to look like human guys, but you're not a man you're a Chicken Boo.
Incest is sorta where I draw the line on being open-minded about relationships and marriage, simply because of how many consequences it carries with it.
What if we add eugenics into the mix? Say Sam and Sally Incest want to get married and have a child. "NAY" screams everyone. Cries of "It's gross" and "Think of the children" echo all around. But let's say the Incests (yes that's what I'm calling them) are taking advantage of cutting technology that allow them to eliminate any and all potential hazards that come from their unholy union? That's right ladies and germs, no defects AT ALL. What say ye? Yuck btw
__________________ Chicken Boo, what's the matter with you? You don't act like the other chickens do. You wear a disguise to look like human guys, but you're not a man you're a Chicken Boo.