Not sure if I'd call the guys my best friends, but there have been 40 or 50 couples that I have been with. Sometimes the guys just sit back & watch, somtimes, they take photos, & videos, sometimes they get involved after I have blasted her with a load of cum & sometimes they are involved from the beginning. Nearly all of my experiences have been positive. The only ones that have bothered me are when the guy was a little too interested in me (I am not even 1% bi). I have done double pussy penetration several times though & it does not bother me, and let me say for the record, when 3 people cum at the same time, it is one hell of a good time and also one hell of a mess...lol
Mostly I believe guys love porn, love to see their wives in porn & of course if the wife is going to be in porn, they definitely want to see her getting impaled my a huge cock regardless of the size of their own.
Of course the women love it as well because they want to enjoy it physically, plus show their man how much they enjoy being fucked by a big one...
Recreational sex for sure, and any good 3rd wheel (or "Bull"), will not act in a way that harms the relationship between a man & woman, but remember he is ther as an enhancement & for fun only...