Not very often. There were unspoken protocols in place.
1. You never "shared" your roommate's casual dates, only committed, trusting partners.
2. You never met with the boyfriend without the roommate present. Ever.
3. If it was obvious that the boyfriend liked you a little "too much," then you never asked or offered to share again.
4. After a breakup, you could only date the ex-boyfriend with explicit consent of the roommate.
It was mostly foolproof. I can only recall once, in 20 years in West Hollywood, New York, and Florida, when it backfired.