This can be one of the most fulfilling part of a mfm threesome. There is just something wild about it. Kind of an additional taboo I guess for men who consider themselves farther on the heterosexual side of spectrum of sexuality.
The first time I ever experienced this was during the first threesome I ever had with a coworker and a guy we met on a business trip. She had already given us both blowjob, I had gone down on her, and we had taken turns fucking her pussy. Then he had fucked her while she took me in her mouth, then we switched. Everyone was so turned on. I couldn’t remember a time when my cock was so hard. It was one of those erections that is so firm it is almost painful.