He’s too chicken-hearted to defend his beliefs in public.

他胆子太小，不敢在公众面前为自己的信仰辩护。

Cheat on somebody:

给某人戴绿帽

例句：

She cheats on her husband.

她给她老公戴绿帽。

Have an affair with somebody:

跟某人有暧昧关系

例句：

You have an affair with him.

你和他有暧昧关系。

Two-time:

对(情人或者配偶)不忠

例句：

Are you sure he’s not two-timing you?

你肯定他没有背着你另有所爱?