Keep in mind that many things are not affected by testosterone. Men can still have orgasms and ejaculate with a flaccid penis. Testosterone does not affect the penile smooth muscles, nor does it help with vascular problems. Even though testosterone is important and does great things, it is not a panacea. Too much can have negative effects.
Large doses can stop sperm production.
Testosterone supplements inhibit the natural production of testosterone. It can change serum cholesterol levels. It can cause damage to the liver. It may increase the risk of male breast and prostate cancer. Men may experience excessive frequency and duration of penile erections.