When I had other relationships, I would tell the basics (we had sex, there was oral, etc) but not the details (first we did it missionary, then switched to cowgirl, it was different than with you because xyz, it was kind of like when I'm with you because of abc, etc). I draw a line at facts unless both partners have previously agreed that they don't mind or like details being shared.
The first guy I dated after deciding to follow a poly path LOVED to hear about my sexual encounters, hubby and I started out swinging and he enjoys hearing about my sexual encounters. Both of them were fine with me sharing whatever kind of information or stories with the other, so boyfriend got stories about hubby and me and whoever else we were with, hubby got stories about how the stories I told boyfriend got him all hot and bothered. All were happy, no discomfort, I was fine with that.
The second guy I was dating had some issues picturing me in bed with others so all he knew was that I was having sex with hubby and one other guy at the time. He didn't want, need, or get details. Hubby and boyfriend did not get details about me with him (he and I didn't have intercourse, but there was some sexual contact so sexual histories were shared).
It all depends on the relationship(s) and the people involved how much can and should be shared.