I have the same fantasy and actually carried it out with my exgf.
In english, this is actually called "Cuckold", and a lot of porn / sex stories are based on this concept, and there seems to be many variations of "cuckold", some involve the bf just watching, or bf gets involved playing 3p, or bf becomes very submissive acting like a slave.
I'm a beta male, and i prefer watching / imagining an Alpha male (strong and good looking man) having sex my gf, and my gf must enjoy the sex.
It's a good one-off thing, and can help to promote sexual relationships, especially when external stimulations are needed to make the sex more interesting / better. But I don't recommend gf being fucked by the same man over and over again, coz u never know when they'll develop real love, and it'll stab your back.
I think the most difficult things are:
i) persuading gf to try it
ii) finding a man that turns gf on, but acceptable to bf to meet his fantasy as well
iii) when different men are needed each time, gf may feel uncomfortable / insecure
iv) how to handle the relationship after it happened once, and
v) make sure the relationship with your gf/ wife, will not solely be based on cuckold, i.e. don't make cuckold the center of your relationship with her.
