So I hope you don't mind, but after reading your post, my mind jumped to a handful of story ideas. The only thing that might have me a little bit nervous about was the cheating wife/gf, though iy wouldn't be so bad if it were a threesome instead. Now, the story ideas:

You and your gf/wife are at the beach when you see a large, beefcake of a man walking down while wearing nothing, but swim trunks. Noticing that you seem to be ogling him almost as much as she is, she decides to spice up their sex life. Now, you two have talked about this before and she is certainly willing to allow another female into the bedroom at least once with her even going out of the way to land an incredibly hot girl, but only if you would be up for letting HER have a "free-pass" on a guy who was impressively built. Reminding him of their agreement, she gets up to go talk to him... (This is where it can branch out into two different options though) Option 1 - The guy agrees to the three way and during the foreplay, the couple finds out just how BIG their guest is. This would naturally lead to small-penis humiliation until finally, they just talk you into being a girl so that you could have more fun. ("Hey, you DID say you wanted a night with two females on one guy...")

The other option here was the guy doesn't end up going home with them, but the wife ends up taking a blue pill instead and winds up a beefy stud. Naturally, she would jump at the chance to compare their sizes, including penis, strength, and even height. This option would lead to much more teasing, especially with small-penis teasing and bi encounter and eventually find yourself taking a pink pill so that you could experience it differently.

The second idea would be more of a straight forward X-change where your male roommate scores a handful of pink pills. The pill-popper could be determined by the person who looses the competition. This could include physical feats, but if it did get out that the roomie is bigger than you, this line WILL come out, "Oh, now you GOTTA take the pill since I'm not being screwed by something so small!" It would most likely be in a joking manner though.

Third option could be brother and sister, but by this point I'd feel like I'm repeating what I said earlier. I hope I at least sparked a grin and a twitch if nothing else!