During medieval times intercourse was part of politics.
I support the notion of seperating friends from lovers. In CK1 any friendship between different sexes was likely to trigger romance and bastard events. Can't I even be friends with my own sister without the game assuming we're having sibling love between the sheets?
I also would like it separated. A while back I proposed a different section for mister/mistresses, and it's basically along the same lines.
On another note, I could see a bad trait of "Dirty Little Secret" about a brother and sister getting it on. That would cause some drama haha.