I don't like things like that either and say so in all of my online profiles when possible.
The only real taboo I engaged in was fucking one of my sisters, who first asked me to after seeing me walk naked & start to jack off late one night, and a few of my cousins when I was younger. Never did it again once we started growing up & going off to college. (But, they do know I live nude whenever possible.) I knew the brothers next door shared a bunk bed until their screened-in porch was remodeled into an extra bedroom; but, I was never sure if they ever played together. 6 or 7 years ago, though, I watched two 19-year-old identical twins who were totally versatile take turns fucking each other.