I fantasize about watching a well-endowed guy fuck my girlfriend.

Haha you're not unique, many men do and many more do but won't admit it.
Try this after a night out and you're in bed getting into it.
Start by telling her how hot she looked, then how many men you noticed checking her out.
Work it, get her firing then whisper in her ear how many of them would love to be in this position.
She will begin to get hot, more urgent movement. Tell her how tight her pussy feels.
Whisper in her ear again, "for him its even tighter as his much bigger cock is fucking your sweet little pussy...."

Wow reading this is a little deja vu, we are no longer together but it had nothing to do with my fantasy. We were together 8 years, she ran track at a very high level in high school, also modeled catwalk and photographic from 18-25, we were in our early 20s when I began to fantasize, sex had gotten dull, my penis is a bit over 6 inches and just over 4 inches in girth, a pencil dick. The fantasy began when after she said she'd never seen a porno I said I'd get one and we'd watch together, We did and during the movie she said it was boring and it did nothing for her yet shortly after it was mind blowing, I could feel the heat before my hand got there and when it did she was incredibly wet, we had passionate sex but said nothing. Every male in the porno was well hung. Things changed in the coming months with me buying her dildos, dirty talking because by now she knew my fantasy and either played to it or genuinely wanted to fulfill it. This led to role playing sexually where we got into dressing up and acting it out, I bought (the first of several) a large penis extension sleeve which she didn't know I was wearing until I began to enter her and the sex was wild and she loved that bigger thicker cock. We discussed a threeway etc many times but I couldn't do it.
We still stay in touch and are good friends, yes she is a size queen, lets say that good looks and personality are first and penis size is a very close second for her. Yes she'll date a good looking guy BUT I know if I run into her somewhere and she introduces him as her partner/boyfriend that he has a very big cock, how do I know for sure? She has enjoyed telling me at times either in a message or on the phone.

I'm a well hung BM and have helped many W couples with this very fantasy. I need to warn you, about 2 out of 3 don't end up going the way they hope.

Some men can't stand to see someone else satisfy their woman. Some women say they want a really big cock, but when it comes time to slide it in (despite lots of foreplay and lube) simply can't accommodate my size and end up disappointed. A few others end up enamored with the size and think an entire relationship can be built around it (it can't).

I even had one woman call me in the middle of the night to tell me she was leaving her husband for me... what? You're leaving your husband of 14 years, your 2-kids, your house in the suburbs just because I have a big dick and got you off a couple times?

Hi we are a w couple she is 5 ft 2 135 hott little blond she has had 10 fat before me I'm small about 5 I would love for her to have something just as big or really a little bigger were are you from we are in northern ky

From a man that has had that same fantasy (and fulfilled it,) I say, if your wife leaves you for this other guy then she didn't love you anyway. We are just talking about sex. Not love. Your wife could fall in love with someone she works with, someone she meets at church, or on the bus for that matter. If she leaves you it's not because of what you did together in bed. It's because she didn't really love you. And why would you want to be with someone like that anyway. If she really loves you and you really love her, and you don't get jealous while it's happening, you'll be fine.

Be very careful. It sometimes happens where guys get aroused at the idea of other men ( with larger cocks ) fucking their woman. But this can lead to your woman desiring the other man more, not being satisfied with you, and losing respect for you. This can lead to you becoming a cuckold. A very dangerous road for a relationship

6.5 inches isn't big that's only average, I'm just on 8inches and at times my wife can't handle the size that the head of my cock get to. I love watching her groan knowing that's she full. I've even had to help our one of her friends because she told her how big I was now she wants me more than her husband. So be careful to what you think you might want if you let her take a larger cock she might not want you anymore

Actually, 6.5 is NOT average, it is roughly the 70 percentile (so 70 percent of all cocks are smaller than 6.5) and that is being conservative; in terms of thickness, 6.5 is in the 95 percentile! You are probably around the 95 percentile in length and there are a variety of studies that used medical personnel and induced erections to back that up. It is unfortunate that the idea of average has gotten so much traction in society; the mode is a far better measurement because it is not affected by outliers and can be stated as a range value without distorting precision or accuracy. The mode would be what most women, bi guys, and gay guys, are likely to encounter when they stick their hand down the front of a pair of pants and that is less than 6 but more than 5.4.
Porn distorts the perspective at several different levels. A professional with the right lens and focal distance can make a 6 inch cock positively huge on a photo and give someone the impression that 8 inches are a dime a dozen. Conversely, the same cock will look considerably smaller when you take a picture from above and particularly if you have strong ligaments which by pulling your penis upwards, shorten the distance between the head and the base thus enhancing the foreshortening effect that is already there.

I don't think it is all that great an idea to let your wife fuck around and worse still if you cannot jump in bed with someone else too but then, if you are going to do all that, why be married? You might be surprised to find that she could care less about your cock size and she might find it annoying that men worry so much about it. Still want to see? There are strapons for men that are in the 10 X 7 range and better but a cock like that would be about 4 standard deviations from the mean!!