My girlfriend thought it would be funny to tie me up and put a metel rod down my penis, is this dangerous because it felt good?

Urethral sounds: Some people who use 'urethral sounds' (metal probes) for sexual pleasure. This can be safe if done carefully with clean equipment, but there is a risk of both infection and urethral injury (irritation or tearing) if things go wrong. If you're going to continue this practice, do it cleanly and carefully.

Yes it is dangerous!: Urethral sounding is a medical procedure that uses probes to locate urethral obstructions ; to ^ urethral diameter. Recreational urethral sounding (in which the sound may be introduced as far as the bladder) is considered a high risk sexual behavior that can cause urethral tears, urethral irritation, internal bleeding ; urinary tract infections which can involve bladder or kidneys. Use of poor >.

Dangerous unless: This is extremely dangerous, unless your girlfriend is a physician or nurse with special training in using sterile procedures and great care not to harm your organs. Otherwise, you could end up needing actual physicians and/or surgeons.