Affair sex is more exciting, forbidden fruit, etc. The Sec. Maybe just ok but the circumstances make it like sex on steroids. Although it's a fantasy that can never last longterm the memories do that's why for most the sex will never be the same in the marriage.
Sounds like after her experience in the A she's just not that into you.
Affairs (especially short ones like she had) are usually nothing more than lust, purely sexual.
Given that she spent 15 years with you first, she had plenty of time to 'get used' to you, for lack of a better term. I've been with my wife about 9 years now, and although the sex is still good to great, it's not the same as it was when we first started dating. "New" sex is exciting. It's what makes things like affairs and ONS's what they are.
She spent 15 years with you, a few months with this guy, then back to you. Technically, it has little to do with you
IMO, but more the general idea of 30 combined years with the same person, and a few months of exciting "new" sex smack dab in the middle. You are not new and exciting (and that's not your fault, obviously). Neither is she.
As said a few times in this thread, sex is pretty much entirely mental. It has little or nothing to do with the AP/OM personally (ie. what he looked like, his penis, etc.) and everything to do with the circumstances (ie. the excitement, the taboo, the 'newness').
When the affair happened, and you insisted on knowing everything (and were told in seemingly great detail... ugh) you put yourself in a position to compete with this guy, and you planted that seed in your wife's head, too. Exacerbated by penis extenders and attempts to get her to orgasm for the past 15 years. Ironically enough, your wife would likely enjoy sex with you much more if it wasn't an obvious attempt to compete with the AP/OM.
But most importantly, your confidence was shot, and never recovered - and your wife KNOWS this. A man who has no confidence in the bedroom will do nothing for a woman. Yes, this is her own doing, but you also have the power to say "**** it" and just do your thing, and not worry about whether she orgasms or not.
Throw away the penis extender thing, it's silly. Your wife probably thinks it's silly, too, but perhaps she doesn't want to tell you for fear of upsetting you. It screams lack of confidence in your manhood (literally and figuratively), and that's a killer for her.