A friend told me his amazing story: he had shared his live-in girlfriend with his three closest male friends, not all at once, but with each individually in MFM threesomes for years. They broke up and he ended up marrying his present wife. He told his wife all about his sexual past with the shared girlfriend, which his wife found thrilling and wanted to experience as well. They didn't, although she and he wanted to do so, as life got in the way. She became pregnant and he assumed that they would never engage in an MFM threesome.
Two months into her pregnancy, she told him that time was running out on them, as she would soon be too big and too near giving birth; and that after giving birth to their child, her vagina would be stretched, her belly covered in stretchmarks, and her breasts too saggy to make a desirable sex partner for his three male friends. He was stunned, but agreed to arrange for one of his friends to join them in their bed that weekend. She had a great time, so another friend joined them in an MFM threesome the weekend following, and finally the last friend of his had his turn at fucking her. Threesomes became their routine weekend fun, until late in her pregnancy, when she had read that too much sex can prompt premature births.
He hoped that a year or two after the birth, she would want to resume the threesomes, but more children came along, so now they wait for the last of their three children to leave for college before taking up threesomes again.