In my opinion, any semen retention is harmful to the sex organs. There is no problem in attempting to prolong an erection without reaching an orgasm, however once an ejaculation is triggered, let it flow as Nature intended.
It seems Tantric is used as a generic term. You may learn from kundalini yoga and Taoist yoga where you can learn to enhance your sexual stamina. Google male deer exercise and kegel exercises. both are well known exercises to increase your sexual stamina arn improve your in erectile dysfunction and premature ejaculation.