Say you are in a long, happy steady relationship and get on very well with a couple of exes - and DP respects that and they also get on well.
And you have a friend who has 2 DCs but has parted from their abusive father, You support her 100% - emotionally, now and then financially, as a friend... and her life choices include (when DC are with father) sleeping with a lot of men.
How would you feel if said friend slept with someone who is both a close friend and an ex; she has also been for lunch with my other closest friend who she only knows through me this week.