I used my roommates toothbrush to clean the toilet
Why would you use your roommates toothbrush on the toilet?
The toothbrush was clean, so I didn't see the problem.
Its not polite to use any of your roommates items without asking them. It is even worse to use their toothbrush to clean the toilet since they will not be able to use that toothbrush anymore. Make sure to at least tell your rooomate about it before the accidentally use it again.
Sure, sure. I guess I was in the wrong here.
Of course you were in the wrong here. Using someone's toothbrush to clean the bathroom is terrible.