Hey Man , you shouldn't worry so much, or in fact at ALL about that!
Listen: I lost my right testicle when I was 12, I am 44 now.
I have had about 12 (longterm) girlfriends and quite some one night stands and I have NEVER met ANY GIRL that had ANY PROBLEM AT ALL with it!! ,or thought of it as strange or whatever!!
Many times it wasn't even mentioned or after making love when talking in bed something like:: Me: "I have only one testicle" Girl: "Oh yeah, I thought I saw that too" end of topic. or: "well it seems to work fine haha" case closed.
Many didn't even see it, even the call girls I were with, or at least I coudn't notice.
Really man, they don't care about a thing like that.
It's not like have only 1 breast or so.
They don't consider it an object they wanna spend much time watching too anyway.
As long as you a capable of a normal erection, (for wich 1 testicle is quite enough) they won't MIND AT ALL.
In fact, the few times I couldn't get it up due to drinking or so, never EVER did a girl come to the stupid idea that it would be because of my 1 testicle, but just what it was: the boozz.
And most of the time, in spite of drinking, my one testicle was truelly enough to be capable to have sex well.
And else, even 5 testicles won't help you there!
Besides if you loose one at the age you did,(as did I) and even later, your other yesticle will start compensating for the lost one so there will be more then enough testosterone!!
It's only in your head man!!
It's not like having a 8cm penis, or looking like Gollem/smeagoll, or smell like a skunk etc etc
So PLEASE STOP torturing YOURSELF for NOTHING, and don't waste your live hiding for the girls, it is SO UNNECESSARY!!
I hope this will will help you, if you have any questions feel free to mail me