I've done the MFM thing quite a few times in a committed relationship, but only done the FMF thing a few times. most women are not all that comfortable with it, aside from fantasy.

I'm not the jealous type, so as long as it's understood that sex is just sex and love is something else, then I'm OK with either type of threesome. Mind you, I have never seen a woman have as many orgasms in one session as with a MFM. With two of us, we can just keep swapping spots if one gets tired or too close to cumming.