I didn't read your other post.
I know a woman who arranged for her best friend to have sex with her husband as a birthday treat...
This is largely stuff of fiction that real people want to emulate because it looks like fun, its a cool thing to do or everybody is doing it...
The truth is everybody is not having threesomes or swinging or stuff like that. These things do not happen naturally very often in real life and when they do it often finishes the relationship or places it under serious pressure.
There are a few success stories - that is inevitable, but they are rare. I also know a few couples who have done a bit of swapping but mostly found it unsatisfying and the prospect was more exciting than the reality.
The main problem you have is sex for a man, especially a young man is mostly physical. His brain is in his balls and he is being driven by the chemicals his body is producing...
It is a thing that goes back to the days when we used to go around on all fours sniffing each others backsides when it was mother nature's job to drive the male to inseminate as many females as he could and the social conventions of our time directly contradict these primitive urges resulting in frustration and discontent.
For women sex mostly has a more emotional content and their primeval urges make them want security and fidelity. Ok that is a bit traditional and the way women behave these days is somewhat at odds with that but once in a relationship it is mostly in a woman's nature to be monogomous but mostly that isn't the natural state for a young man so they don't see these things in the same way as women.
It takes a very strong relationship between some really understanding people to have a threesome or do a spot of swapping and then continue as before - OR it needs a couple who really aren't that close to begin with...
The people who can completely separate their emotional life from their sex life are very rare.
How would your boyfriend react, I wonder, if you agreed and was really enthusiastic about the prospect of sex with another man?
I don't recommend you find another girl for your boyfriend unless you actually want to do that. If you are pressured into it, it would be wrong to do it and not a very good basis for any long-term relationship.
If your boyfriend is feeling inadequate about his performance I can't see introducing another man into the picture is going to help.
If your boyfriend is inexperienced and insecure and he merely wants to learn by watching another man's performance...
Have you thought about the possibility of latent homosexuality, bisexuality or perhaps he has voyueristic tendencies?
Is introducing another man into the relationship for your benefit or really for his benefit or for both?
Seems to me the two of you should talk honestly and at length about this - certainly neither of you should consider a threesome, foursome or an orgy as a way of fixing anything - chances are if you fixed your relationship with each other the subject of threesomes would not have arisen in the first place...
Perhaps it is a good thing to try everything once but it is definitely a bad thing to be forced to do something you don't want to do!