Heather Corinna replies:
I was born with an undescending testicle that was removed at the age of 3. It has made me pretty unconfident when engaging in sexual intercourse, which lately has been affecting my performance. I just want to know if only having one would affect sustaining a good erection, testerone levels for development, or the chances of reproducing?
Having one testicle has absolutely no physiological impact on sexual performance. Arousal, erection and orgasm are not reliant on the testicles in any way. As well, having your testicle removed, and but one remaining would not have impacted your sexual or general development, and your fertility is very likely just as good as anyone else's: if you had had two undescended, that can present problems (with fertility, development and performance), but it is not known to be problematic because of just one.
But for certain, big body image issues and feeling unconfident about having one testicle can impact all of those things: negative body image is a big sexual blockade for men and women alike. It's pretty normal to feel unconfident and concerned when you have a very visible genital variance: sex can make us feel vulnerable and overexposed enough when we look like everyone else, but when there is something different about us, it's unsurprising for us to feel even more so.
Chances are, it's those feelings creating any erection or performance problems, not a physical issue. By all means, you can double-check with your doctor, but if they echo the same response, what I'd suggest is doing some work on your body image for yourself (and if you feel like some counseling would help, by all means, set that up), making some peace with your solo guy there, and just making sure you're with partners who you feel overall as secure as can be with, who you know accept you just as you are. Ultimately, looking through some reports, I can tell you that it's pretty clear most partners -- of any sex or gender -- really don't care about someone having one testicle or two.
Here's a really nice additional answer on a similar issue at Go Ask Alice which may be of some comfort, too.