I happen to agree with that. There's a certain point in which a couple may decide that their intention is a long-term commitment, sometimes for the remainder of their lives. Indeed, a couple like that needs the blessing of both sets of parents.
Three-way as in just a one time thing or an actual relationship?
The latter can be extremely complicated. I may have started a story about something like that.
I happen to know that, although my mom will always have misgivings, she'll always support me no matter what decision I make. I've only told her part of the truth. I know she will have a hard time. As for friends, if they don't accept it, they're not true friends, unless there is something they know that they're not telling me.