Your an ass. If you thought you were mature enough to get married, then you should have been mature enough to realize the responsibility and commitment of a marriage means not only putting your interests first, but to also consider your partner's.
Putting her health in danger by screwing around is soooo discourteous. Condoms don't prevent all STD's.
If she knows and doesn't care, then she has little self concern.
If it's an open marriage, and the monogamy part has been omitted, why bother getting married in the first place, unless you're looking for tax breaks, health insurance, or a cash horse.
That's why I won't get married; The only person I want to be responsible for or concerned with is myself.