I did most of the talking with my older 2, the Ex was a worthless PDN, and I didnt want him telling my boys ANYTHING about that.....Ive had a LOT of talks in the past 2 years or so with oldest (18) about like Nancy said, the feelings, girl POV and consequences since he has a steady GF now.
We both talked to his son when we found out he was having sex with his hoochy GF, and the twins are too little to talk about more than why they're too young to "date" although they both have boyfriends/girlfriends in first grade

.....but as they get older it'll probably be me that tells them most of the deets. BF's not much of a talker about that kind of stuff unless he has to.