1. To have rough, angry sex with a woman you find very attractive, but you hold back a little bit to ensure the integrity of the condom as insurance so you do not have to pay child support or spend any unnecessary time outside of the bedroom together.
2. To be brought unwillingly into a more serious phase of a relationship with a parter you would rather have as a causal sex buddy through any means.
To Break the Trojan would mean the girl would become impregnated and you would either have to marry her, or pay child support for 18 years.
Guy 1: Man Michelle is smokin hot, would you hit that?
Guy 2: Yeah I'd hit that, with a vengeance, but I wouldn't Break the Trojan.
Guy 1: Word, I hear shes an alcoholic anyways.