In the sim? I'm all up for that, gives more potential.
IRL, I think the common reaction of guys is mostly 'eww, guys, that's gaaay', although I think that's just socially-instilled homophobia rather than anything overly impractical or disgusting about sharing a lady with another guy.
If anything, MMF threesomes are more practical than MFF, as with two girls often the guy finishes early with the double stimulation and the girls get more into each other (or just the girls get into each other and the guy gets left out!) With two guys, the girl gets double the satisfaction and the guys can ignore each other if they want.
For me, it'd have to be someone I'm pretty open with (only a couple of guys I can think of, with very similar taste to me), or, say, some celebrity that your lady really likes and the two of you get the chance to seduce as a favour to her - lets you suggest a girl-girl threesome at another time :p.
As Lady Gaga and Justin Timberlake said on that SNL skit, 'it's not gay if it's a 3-way!'