In my experience (long but not very extensive) it's tended not to be casual, simply becausei t took so long to persuade her!

Last year however a friend and I did it on the just good friends plus sex basis. Once. It might of been more but I found someone I cared for and so id she.   Wer'e still friends.

As long as your both grownup and sensible, do what you want. ANd make sure she comes first!